other computer languages. Mastering even the basics of regular
expressions will allow you to manipulate text with surprising ease.
-What is a regular expression? A regular expression is simply a string
-that describes a pattern. Patterns are in common use these days;
+What is a regular expression? At its most basic, a regular expression
+is a template that is used to determine if a string has certain
+characteristics. The string is most often some text, such as a line,
+sentence, web page, or even a whole book, but less commonly it could be
+some binary data as well.
+Suppose we want to determine if the text in variable, C<$var> contains
+the sequence of characters S<C<m u s h r o o m>>
+(blanks added for legibility). We can write in Perl
+
+ $var =~ m/mushroom/
+
+The value of this expression will be TRUE if C<$var> contains that
+sequence of characters, and FALSE otherwise. The portion enclosed in
+C<'E<sol>'> characters denotes the characteristic we are looking for.
+We use the term I<pattern> for it. The process of looking to see if the
+pattern occurs in the string is called I<matching>, and the C<"=~">
+operator along with the C<m//> tell Perl to try to match the pattern
+against the string. Note that the pattern is also a string, but a very
+special kind of one, as we will see. Patterns are in common use these
+days;
examples are the patterns typed into a search engine to find web pages
-and the patterns used to list files in a directory, e.g., C<ls *.txt>
-or C<dir *.*>. In Perl, the patterns described by regular expressions
-are used to search strings, extract desired parts of strings, and to
-do search and replace operations.
+and the patterns used to list files in a directory, I<e.g.>, "C<ls *.txt>"
+or "C<dir *.*>". In Perl, the patterns described by regular expressions
+are used not only to search strings, but to also extract desired parts
+of strings, and to do search and replace operations.
Regular expressions have the undeserved reputation of being abstract
-and difficult to understand. Regular expressions are constructed using
+and difficult to understand. This really stems simply because the
+notation used to express them tends to be terse and dense, and not
+because of inherent complexity. We recommend using the C</x> regular
+expression modifier (described below) along with plenty of white space
+to make them less dense, and easier to read. Regular expressions are
+constructed using
simple concepts like conditionals and loops and are no more difficult
to understand than the corresponding C<if> conditionals and C<while>
-loops in the Perl language itself. In fact, the main challenge in
-learning regular expressions is just getting used to the terse
-notation used to express these concepts.
+loops in the Perl language itself.
This tutorial flattens the learning curve by discussing regular
expression concepts, along with their notation, one at a time and with
discusses the more advanced regular expression operators and
introduces the latest cutting-edge innovations.
-A note: to save time, 'regular expression' is often abbreviated as
+A note: to save time, "regular expression" is often abbreviated as
regexp or regex. Regexp is a more natural abbreviation than regex, but
is harder to pronounce. The Perl pod documentation is evenly split on
regexp vs regex; in Perl, there is more than one way to abbreviate it.
We'll use regexp in this tutorial.
+New in v5.22, L<C<use re 'strict'>|re/'strict' mode> applies stricter
+rules than otherwise when compiling regular expression patterns. It can
+find things that, while legal, may not be what you intended.
+
=head1 Part 1: The basics
=head2 Simple word matching
The simplest regexp is simply a word, or more generally, a string of
-characters. A regexp consisting of a word matches any string that
+characters. A regexp consisting of just a word matches any string that
contains that word:
"Hello World" =~ /World/; # matches
The literal string in the regexp can be replaced by a variable:
- $greeting = "World";
+ my $greeting = "World";
if ("Hello World" =~ /$greeting/) {
print "It matches\n";
}
# '/' becomes an ordinary char
C</World/>, C<m!World!>, and C<m{World}> all represent the
-same thing. When, e.g., the quote (C<">) is used as a delimiter, the forward
+same thing. When, I<e.g.>, the quote (C<'"'>) is used as a delimiter, the forward
slash C<'/'> becomes an ordinary character and can be used in this regexp
without trouble.
The first regexp C<world> doesn't match because regexps are
case-sensitive. The second regexp matches because the substring
S<C<'o W'>> occurs in the string S<C<"Hello World">>. The space
-character ' ' is treated like any other character in a regexp and is
+character C<' '> is treated like any other character in a regexp and is
needed to match in this case. The lack of a space character is the
reason the third regexp C<'oW'> doesn't match. The fourth regexp
-C<'World '> doesn't match because there is a space at the end of the
+"C<World >" doesn't match because there is a space at the end of the
regexp, but not at the end of the string. The lesson here is that
regexps must match a part of the string I<exactly> in order for the
statement to be true.
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
With respect to character matching, there are a few more points you
-need to know about. First of all, not all characters can be used 'as
-is' in a match. Some characters, called I<metacharacters>, are reserved
+need to know about. First of all, not all characters can be used "as
+is" in a match. Some characters, called I<metacharacters>, are reserved
for use in regexp notation. The metacharacters are
- {}[]()^$.|*+?\
+ {}[]()^$.|*+?-\
The significance of each of these will be explained
in the rest of the tutorial, but for now, it is important only to know
-that a metacharacter can be matched by putting a backslash before it:
+that a metacharacter can be matched as-is by putting a backslash before
+it:
"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
"2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
'C:\WIN32' =~ /C:\\WIN/; # matches
+In situations where it doesn't make sense for a particular metacharacter
+to mean what it normally does, it automatically loses its
+metacharacter-ness and becomes an ordinary character that is to be
+matched literally. For example, the C<'}'> is a metacharacter only when
+it is the mate of a C<'{'> metacharacter. Otherwise it is treated as a
+literal RIGHT CURLY BRACKET. This may lead to unexpected results.
+L<C<use re 'strict'>|re/'strict' mode> can catch some of these.
+
In addition to the metacharacters, there are some ASCII characters
which don't have printable character equivalents and are instead
represented by I<escape sequences>. Common examples are C<\t> for a
tab, C<\n> for a newline, C<\r> for a carriage return and C<\a> for a
bell (or alert). If your string is better thought of as a sequence of arbitrary
-bytes, the octal escape sequence, e.g., C<\033>, or hexadecimal escape
-sequence, e.g., C<\x1B> may be a more natural representation for your
+bytes, the octal escape sequence, I<e.g.>, C<\033>, or hexadecimal escape
+sequence, I<e.g.>, C<\x1B> may be a more natural representation for your
bytes. Here are some examples of escapes:
"1000\t2000" =~ m(0\t2) # matches
With all of the regexps above, if the regexp matched anywhere in the
string, it was considered a match. Sometimes, however, we'd like to
specify I<where> in the string the regexp should try to match. To do
-this, we would use the I<anchor> metacharacters C<^> and C<$>. The
-anchor C<^> means match at the beginning of the string and the anchor
-C<$> means match at the end of the string, or before a newline at the
+this, we would use the I<anchor> metacharacters C<'^'> and C<'$'>. The
+anchor C<'^'> means match at the beginning of the string and the anchor
+C<'$'> means match at the end of the string, or before a newline at the
end of the string. Here is how they are used:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /keeper$/; # matches
"housekeeper\n" =~ /keeper$/; # matches
-The second regexp doesn't match because C<^> constrains C<keeper> to
+The second regexp doesn't match because C<'^'> constrains C<keeper> to
match only at the beginning of the string, but C<"housekeeper"> has
keeper starting in the middle. The third regexp does match, since the
-C<$> constrains C<keeper> to match only at the end of the string.
+C<'$'> constrains C<keeper> to match only at the end of the string.
-When both C<^> and C<$> are used at the same time, the regexp has to
-match both the beginning and the end of the string, i.e., the regexp
+When both C<'^'> and C<'$'> are used at the same time, the regexp has to
+match both the beginning and the end of the string, I<i.e.>, the regexp
matches the whole string. Consider
"keeper" =~ /^keep$/; # doesn't match
The first regexp doesn't match because the string has more to it than
C<keep>. Since the second regexp is exactly the string, it
-matches. Using both C<^> and C<$> in a regexp forces the complete
+matches. Using both C<'^'> and C<'$'> in a regexp forces the complete
string to match, so it gives you complete control over which strings
match and which don't. Suppose you are looking for a fellow named
bert, off in a string by himself:
One such concept is that of a I<character class>. A character class
allows a set of possible characters, rather than just a single
-character, to match at a particular point in a regexp. Character
-classes are denoted by brackets C<[...]>, with the set of characters
+character, to match at a particular point in a regexp. You can define
+your own custom character classes. These
+are denoted by brackets C<[...]>, with the set of characters
to be possibly matched inside. Here are some examples:
/cat/; # matches 'cat'
We saw in the section above that there were ordinary characters, which
represented themselves, and special characters, which needed a
-backslash C<\> to represent themselves. The same is true in a
+backslash C<'\'> to represent themselves. The same is true in a
character class, but the sets of ordinary and special characters
inside a character class are different than those outside a character
class. The special characters for a character class are C<-]\^$> (and
the pattern delimiter, whatever it is).
-C<]> is special because it denotes the end of a character class. C<$> is
-special because it denotes a scalar variable. C<\> is special because
+C<']'> is special because it denotes the end of a character class. C<'$'> is
+special because it denotes a scalar variable. C<'\'> is special because
it is used in escape sequences, just like above. Here is how the
special characters C<]$\> are handled:
/[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
The last two are a little tricky. In C<[\$x]>, the backslash protects
-the dollar sign, so the character class has two members C<$> and C<x>.
+the dollar sign, so the character class has two members C<'$'> and C<'x'>.
In C<[\\$x]>, the backslash is protected, so C<$x> is treated as a
variable and substituted in double quote fashion.
treated as an ordinary character; C<[-ab]>, C<[ab-]> and C<[a\-b]> are
all equivalent.
-The special character C<^> in the first position of a character class
+The special character C<'^'> in the first position of a character class
denotes a I<negated character class>, which matches any character but
those in the brackets. Both C<[...]> and C<[^...]> must match a
character, or the match fails. Then
Now, even C<[0-9]> can be a bother to write multiple times, so in the
interest of saving keystrokes and making regexps more readable, Perl
has several abbreviations for common character classes, as shown below.
-Since the introduction of Unicode, unless the C<//a> modifier is in
+Since the introduction of Unicode, unless the C</a> modifier is in
effect, these character classes match more than just a few characters in
the ASCII range.
=item *
-\d matches a digit, not just [0-9] but also digits from non-roman scripts
+C<\d> matches a digit, not just C<[0-9]> but also digits from non-roman scripts
=item *
-\s matches a whitespace character, the set [\ \t\r\n\f] and others
+C<\s> matches a whitespace character, the set C<[\ \t\r\n\f]> and others
=item *
-\w matches a word character (alphanumeric or _), not just [0-9a-zA-Z_]
+C<\w> matches a word character (alphanumeric or C<'_'>), not just C<[0-9a-zA-Z_]>
but also digits and characters from non-roman scripts
=item *
-\D is a negated \d; it represents any other character than a digit, or [^\d]
+C<\D> is a negated C<\d>; it represents any other character than a digit, or C<[^\d]>
=item *
-\S is a negated \s; it represents any non-whitespace character [^\s]
+C<\S> is a negated C<\s>; it represents any non-whitespace character C<[^\s]>
=item *
-\W is a negated \w; it represents any non-word character [^\w]
+C<\W> is a negated C<\w>; it represents any non-word character C<[^\w]>
=item *
-The period '.' matches any character but "\n" (unless the modifier C<//s> is
+The period C<'.'> matches any character but C<"\n"> (unless the modifier C</s> is
in effect, as explained below).
=item *
-\N, like the period, matches any character but "\n", but it does so
-regardless of whether the modifier C<//s> is in effect.
+C<\N>, like the period, matches any character but C<"\n">, but it does so
+regardless of whether the modifier C</s> is in effect.
=back
-The C<//a> modifier, available starting in Perl 5.14, is used to
-restrict the matches of \d, \s, and \w to just those in the ASCII range.
+The C</a> modifier, available starting in Perl 5.14, is used to
+restrict the matches of C<\d>, C<\s>, and C<\w> to just those in the ASCII range.
It is useful to keep your program from being needlessly exposed to full
Unicode (and its accompanying security considerations) when all you want
-is to process English-like text. (The "a" may be doubled, C<//aa>, to
+is to process English-like text. (The "a" may be doubled, C</aa>, to
provide even more restrictions, preventing case-insensitive matching of
ASCII with non-ASCII characters; otherwise a Unicode "Kelvin Sign"
would caselessly match a "k" or "K".)
The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside
-of character classes. Here are some in use:
+of bracketed character classes. Here are some in use:
/\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
/[\d\s]/; # matches any digit or whitespace character
fact C<[^\d\w]> is the same as C<[^\w]>, which is the same as
C<[\W]>. Think DeMorgan's laws.
+In actuality, the period and C<\d\s\w\D\S\W> abbreviations are
+themselves types of character classes, so the ones surrounded by
+brackets are just one type of character class. When we need to make a
+distinction, we refer to them as "bracketed character classes."
+
An anchor useful in basic regexps is the I<word anchor>
C<\b>. This matches a boundary between a word character and a non-word
character C<\w\W> or C<\W\w>:
Note in the last example, the end of the string is considered a word
boundary.
+For natural language processing (so that, for example, apostrophes are
+included in words), use instead C<\b{wb}>
+
+ "don't" =~ / .+? \b{wb} /x; # matches the whole string
+
You might wonder why C<'.'> matches everything but C<"\n"> - why not
every character? The reason is that often one is matching against
lines and would like to ignore the newline characters. For instance,
This behavior is convenient, because we usually want to ignore
newlines when we count and match characters in a line. Sometimes,
-however, we want to keep track of newlines. We might even want C<^>
-and C<$> to anchor at the beginning and end of lines within the
+however, we want to keep track of newlines. We might even want C<'^'>
+and C<'$'> to anchor at the beginning and end of lines within the
string, rather than just the beginning and end of the string. Perl
allows us to choose between ignoring and paying attention to newlines
-by using the C<//s> and C<//m> modifiers. C<//s> and C<//m> stand for
+by using the C</s> and C</m> modifiers. C</s> and C</m> stand for
single line and multi-line and they determine whether a string is to
be treated as one continuous string, or as a set of lines. The two
modifiers affect two aspects of how the regexp is interpreted: 1) how
-the C<'.'> character class is defined, and 2) where the anchors C<^>
-and C<$> are able to match. Here are the four possible combinations:
+the C<'.'> character class is defined, and 2) where the anchors C<'^'>
+and C<'$'> are able to match. Here are the four possible combinations:
=over 4
=item *
-no modifiers (//): Default behavior. C<'.'> matches any character
-except C<"\n">. C<^> matches only at the beginning of the string and
-C<$> matches only at the end or before a newline at the end.
+no modifiers: Default behavior. C<'.'> matches any character
+except C<"\n">. C<'^'> matches only at the beginning of the string and
+C<'$'> matches only at the end or before a newline at the end.
=item *
-s modifier (//s): Treat string as a single long line. C<'.'> matches
-any character, even C<"\n">. C<^> matches only at the beginning of
-the string and C<$> matches only at the end or before a newline at the
+s modifier (C</s>): Treat string as a single long line. C<'.'> matches
+any character, even C<"\n">. C<'^'> matches only at the beginning of
+the string and C<'$'> matches only at the end or before a newline at the
end.
=item *
-m modifier (//m): Treat string as a set of multiple lines. C<'.'>
-matches any character except C<"\n">. C<^> and C<$> are able to match
+m modifier (C</m>): Treat string as a set of multiple lines. C<'.'>
+matches any character except C<"\n">. C<'^'> and C<'$'> are able to match
at the start or end of I<any> line within the string.
=item *
-both s and m modifiers (//sm): Treat string as a single long line, but
+both s and m modifiers (C</sm>): Treat string as a single long line, but
detect multiple lines. C<'.'> matches any character, even
-C<"\n">. C<^> and C<$>, however, are able to match at the start or end
+C<"\n">. C<'^'> and C<'$'>, however, are able to match at the start or end
of I<any> line within the string.
=back
-Here are examples of C<//s> and C<//m> in action:
+Here are examples of C</s> and C</m> in action:
$x = "There once was a girl\nWho programmed in Perl\n";
$x =~ /girl.Who/m; # doesn't match, "." doesn't match "\n"
$x =~ /girl.Who/sm; # matches, "." matches "\n"
-Most of the time, the default behavior is what is wanted, but C<//s> and
-C<//m> are occasionally very useful. If C<//m> is being used, the start
+Most of the time, the default behavior is what is wanted, but C</s> and
+C</m> are occasionally very useful. If C</m> is being used, the start
of the string can still be matched with C<\A> and the end of the string
can still be matched with the anchors C<\Z> (matches both the end and
-the newline before, like C<$>), and C<\z> (matches only the end):
+the newline before, like C<'$'>), and C<\z> (matches only the end):
$x =~ /^Who/m; # matches, "Who" at start of second line
$x =~ /\AWho/m; # doesn't match, "Who" is not at start of string
Sometimes we would like our regexp to be able to match different
possible words or character strings. This is accomplished by using
-the I<alternation> metacharacter C<|>. To match C<dog> or C<cat>, we
+the I<alternation> metacharacter C<'|'>. To match C<dog> or C<cat>, we
form the regexp C<dog|cat>. As before, Perl will try to match the
regexp at the earliest possible point in the string. At each
character position, Perl will first try to match the first
The process of trying one alternative, seeing if it matches, and
moving on to the next alternative, while going back in the string
from where the previous alternative was tried, if it doesn't, is called
-I<backtracking>. The term 'backtracking' comes from the idea that
+I<backtracking>. The term "backtracking" comes from the idea that
matching a regexp is like a walk in the woods. Successfully matching
a regexp is like arriving at a destination. There are many possible
trailheads, one for each string position, and each one is tried in
=over 4
-=item 0
-
-Start with the first letter in the string 'a'.
+=item Z<>0. Start with the first letter in the string C<'a'>.
-=item 1
+E<nbsp>
-Try the first alternative in the first group 'abd'.
+=item Z<>1. Try the first alternative in the first group C<'abd'>.
-=item 2
+E<nbsp>
-Match 'a' followed by 'b'. So far so good.
+=item Z<>2. Match C<'a'> followed by C<'b'>. So far so good.
-=item 3
+E<nbsp>
-'d' in the regexp doesn't match 'c' in the string - a dead
-end. So backtrack two characters and pick the second alternative in
-the first group 'abc'.
+=item Z<>3. C<'d'> in the regexp doesn't match C<'c'> in the string - a
+dead end. So backtrack two characters and pick the second alternative
+in the first group C<'abc'>.
-=item 4
+E<nbsp>
-Match 'a' followed by 'b' followed by 'c'. We are on a roll
-and have satisfied the first group. Set $1 to 'abc'.
+=item Z<>4. Match C<'a'> followed by C<'b'> followed by C<'c'>. We are on a roll
+and have satisfied the first group. Set C<$1> to C<'abc'>.
-=item 5
+E<nbsp>
-Move on to the second group and pick the first alternative
-'df'.
+=item Z<>5 Move on to the second group and pick the first alternative C<'df'>.
-=item 6
+E<nbsp>
-Match the 'd'.
+=item Z<>6 Match the C<'d'>.
-=item 7
+E<nbsp>
-'f' in the regexp doesn't match 'e' in the string, so a dead
+=item Z<>7. C<'f'> in the regexp doesn't match C<'e'> in the string, so a dead
end. Backtrack one character and pick the second alternative in the
-second group 'd'.
+second group C<'d'>.
-=item 8
+E<nbsp>
-'d' matches. The second grouping is satisfied, so set $2 to
-'d'.
+=item Z<>8. C<'d'> matches. The second grouping is satisfied, so set
+C<$2> to C<'d'>.
-=item 9
+E<nbsp>
-We are at the end of the regexp, so we are done! We have
-matched 'abcd' out of the string "abcde".
+=item Z<>9. We are at the end of the regexp, so we are done! We have
+matched C<'abcd'> out of the string C<"abcde">.
=back
There are a couple of things to note about this analysis. First, the
-third alternative in the second group 'de' also allows a match, but we
+third alternative in the second group C<'de'> also allows a match, but we
stopped before we got to it - at a given character position, leftmost
wins. Second, we were able to get a match at the first character
-position of the string 'a'. If there were no matches at the first
-position, Perl would move to the second character position 'b' and
+position of the string C<'a'>. If there were no matches at the first
+position, Perl would move to the second character position C<'b'> and
attempt the match all over again. Only when all possible paths at all
possible character positions have been exhausted does Perl give
up and declare S<C<$string =~ /(abd|abc)(df|d|de)/;>> to be false.
different function: they allow the extraction of the parts of a string
that matched. This is very useful to find out what matched and for
text processing in general. For each grouping, the part that matched
-inside goes into the special variables C<$1>, C<$2>, etc. They can be
+inside goes into the special variables C<$1>, C<$2>, I<etc>. They can be
used just as ordinary variables:
# extract hours, minutes, seconds
If the groupings in a regexp are nested, C<$1> gets the group with the
leftmost opening parenthesis, C<$2> the next opening parenthesis,
-etc. Here is a regexp with nested groups:
+I<etc>. Here is a regexp with nested groups:
/(ab(cd|ef)((gi)|j))/;
1 2 34
For convenience, Perl sets C<$+> to the string held by the highest numbered
C<$1>, C<$2>,... that got assigned (and, somewhat related, C<$^N> to the
-value of the C<$1>, C<$2>,... most-recently assigned; i.e. the C<$1>,
+value of the C<$1>, C<$2>,... most-recently assigned; I<i.e.> the C<$1>,
C<$2>,... associated with the rightmost closing parenthesis used in the
match).
matching variables that can be used I<inside> a regexp. This is a
really nice feature; what matches later in a regexp is made to depend on
what matched earlier in the regexp. Suppose we wanted to look
-for doubled words in a text, like 'the the'. The following regexp finds
+for doubled words in a text, like "the the". The following regexp finds
all 3-letter doubles with a space in between:
/\b(\w\w\w)\s\g1\b/;
-The grouping assigns a value to \g1, so that the same 3-letter sequence
+The grouping assigns a value to C<\g1>, so that the same 3-letter sequence
is used for both parts.
A similar task is to find words consisting of two identical parts:
papa
The regexp has a single grouping which considers 4-letter
-combinations, then 3-letter combinations, etc., and uses C<\g1> to look for
+combinations, then 3-letter combinations, I<etc>., and uses C<\g1> to look for
a repeat. Although C<$1> and C<\g1> represent the same thing, care should be
taken to use matched variables C<$1>, C<$2>,... only I<outside> a regexp
and backreferences C<\g1>, C<\g2>,... only I<inside> a regexp; not doing
=head2 Relative backreferences
Counting the opening parentheses to get the correct number for a
-backreference is errorprone as soon as there is more than one
+backreference is error-prone as soon as there is more than one
capturing group. A more convenient technique became available
with Perl 5.10: relative backreferences. To refer to the immediately
preceding capture group one now may write C<\g{-1}>, the next but
Assuming that we have to match calendar dates which may be given in one
of the three formats yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy, we can write
-three suitable patterns where we use 'd', 'm' and 'y' respectively as the
+three suitable patterns where we use C<'d'>, C<'m'> and C<'y'> respectively as the
names of the groups capturing the pertaining components of a date. The
matching operation combines the three patterns as alternatives:
set around an alternative achieves. Here is an extended version of the
previous pattern:
- if ( $time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/ ){
- print "hour=$1 minute=$2 zone=$3\n";
- }
+ if($time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/){
+ print "hour=$1 minute=$2 zone=$3\n";
+ }
Within the alternative numbering group, group numbers start at the same
position for each alternative. After the group, numbering continues
=head2 Position information
-In addition to what was matched, Perl (since 5.6.0) also provides the
+In addition to what was matched, Perl also provides the
positions of what was matched as contents of the C<@-> and C<@+>
arrays. C<$-[0]> is the position of the start of the entire match and
C<$+[0]> is the position of the end. Similarly, C<$-[n]> is the
$x = "Mmm...donut, thought Homer";
$x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
- foreach $expr (1..$#-) {
- print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
+ foreach $exp (1..$#-) {
+ print "Match $exp: '${$exp}' at position ($-[$exp],$+[$exp])\n";
}
prints
Even if there are no groupings in a regexp, it is still possible to
find out what exactly matched in a string. If you use them, Perl
will set C<$`> to the part of the string before the match, will set C<$&>
-to the part of the string that matched, and will set C<$'> to the part
+to the part of the string that matched, and will set C<'$'> to the part
of the string after the match. An example:
$x = "the cat caught the mouse";
In the second match, C<$`> equals C<''> because the regexp matched at the
first character position in the string and stopped; it never saw the
-second 'the'. It is important to note that using C<$`> and C<$'>
+second "the".
+
+If your code is to run on Perl versions earlier than
+5.20, it is worthwhile to note that using C<$`> and C<'$'>
slows down regexp matching quite a bit, while C<$&> slows it down to a
lesser extent, because if they are used in one regexp in a program,
they are generated for I<all> regexps in the program. So if raw
$' is the same as substr( $x, $+[0] )
As of Perl 5.10, the C<${^PREMATCH}>, C<${^MATCH}> and C<${^POSTMATCH}>
-variables may be used. These are only set if the C</p> modifier is present.
-Consequently they do not penalize the rest of the program.
+variables may be used. These are only set if the C</p> modifier is
+present. Consequently they do not penalize the rest of the program. In
+Perl 5.20, C<${^PREMATCH}>, C<${^MATCH}> and C<${^POSTMATCH}> are available
+whether the C</p> has been used or not (the modifier is ignored), and
+C<$`>, C<'$'> and C<$&> do not cause any speed difference.
=head2 Non-capturing groupings
@num = split /(a|b)+/, $x; # @num = ('12','a','34','a','5')
@num = split /(?:a|b)+/, $x; # @num = ('12','34','5')
+In Perl 5.22 and later, all groups within a regexp can be set to
+non-capturing by using the new C</n> flag:
+
+ "hello" =~ /(hi|hello)/n; # $1 is not set!
+
+See L<perlre/"n"> for more information.
=head2 Matching repetitions
of any length, without writing out tedious alternatives like
C<\w\w\w\w|\w\w\w|\w\w|\w>.
-This is exactly the problem the I<quantifier> metacharacters C<?>,
-C<*>, C<+>, and C<{}> were created for. They allow us to delimit the
+This is exactly the problem the I<quantifier> metacharacters C<'?'>,
+C<'*'>, C<'+'>, and C<{}> were created for. They allow us to delimit the
number of repeats for a portion of a regexp we consider to be a
match. Quantifiers are put immediately after the character, character
class, or grouping that we want to specify. They have the following
=item *
-C<a?> means: match 'a' 1 or 0 times
+C<a?> means: match C<'a'> 1 or 0 times
=item *
-C<a*> means: match 'a' 0 or more times, i.e., any number of times
+C<a*> means: match C<'a'> 0 or more times, I<i.e.>, any number of times
=item *
-C<a+> means: match 'a' 1 or more times, i.e., at least once
+C<a+> means: match C<'a'> 1 or more times, I<i.e.>, at least once
=item *
/y(es)?/i; # matches 'y', 'Y', or a case-insensitive 'yes'
$year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more
# than 4 digits
- $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3-digit dates
- $year =~ /^\d{2}(\d{2})?$/; # same thing written differently. However,
- # this captures the last two digits in $1
- # and the other does not.
+ $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3-digit dates
+ $year =~ /^\d{2}(\d{2})?$/; # same thing written differently.
+ # However, this captures the last two
+ # digits in $1 and the other does not.
% simple_grep '^(\w+)\g1$' /usr/dict/words # isn't this easier?
beriberi
For all of these quantifiers, Perl will try to match as much of the
string as possible, while still allowing the regexp to succeed. Thus
-with C</a?.../>, Perl will first try to match the regexp with the C<a>
+with C</a?.../>, Perl will first try to match the regexp with the C<'a'>
present; if that fails, Perl will try to match the regexp without the
-C<a> present. For the quantifier C<*>, we get the following:
+C<'a'> present. For the quantifier C<'*'>, we get the following:
$x = "the cat in the hat";
$x =~ /^(.*)(cat)(.*)$/; # matches,
=item *
-Principle 2: The maximal matching quantifiers C<?>, C<*>, C<+> and
+Principle 2: The maximal matching quantifiers C<'?'>, C<'*'>, C<'+'> and
C<{n,m}> will in general match as much of the string as possible while
still allowing the whole regexp to match.
# $3 = 'l'
This regexp matches at the earliest string position, C<'T'>. One
-might think that C<e>, being leftmost in the alternation, would be
-matched, but C<r> produces the longest string in the first quantifier.
+might think that C<'e'>, being leftmost in the alternation, would be
+matched, but C<'r'> produces the longest string in the first quantifier.
$x =~ /(m{1,2})(.*)$/; # matches,
# $1 = 'mm'
Here, C<.?> eats its maximal one character at the earliest possible
position in the string, C<'a'> in C<programming>, leaving C<m{1,2}>
-the opportunity to match both C<m>'s. Finally,
+the opportunity to match both C<'m'>'s. Finally,
"aXXXb" =~ /(X*)/; # matches with $1 = ''
match a I<minimal> piece of string, rather than a maximal piece. For
this purpose, Larry Wall created the I<minimal match> or
I<non-greedy> quantifiers C<??>, C<*?>, C<+?>, and C<{}?>. These are
-the usual quantifiers with a C<?> appended to them. They have the
+the usual quantifiers with a C<'?'> appended to them. They have the
following meanings:
=over 4
=item *
-C<a??> means: match 'a' 0 or 1 times. Try 0 first, then 1.
+C<a??> means: match C<'a'> 0 or 1 times. Try 0 first, then 1.
=item *
-C<a*?> means: match 'a' 0 or more times, i.e., any number of times,
+C<a*?> means: match C<'a'> 0 or more times, I<i.e.>, any number of times,
but as few times as possible
=item *
-C<a+?> means: match 'a' 1 or more times, i.e., at least once, but
+C<a+?> means: match C<'a'> 1 or more times, I<i.e.>, at least once, but
as few times as possible
=item *
# $2 = 'e'
# $3 = ' programming republic of Perl'
-The minimal string that will allow both the start of the string C<^>
+The minimal string that will allow both the start of the string C<'^'>
and the alternation to match is C<Th>, with the alternation C<e|r>
-matching C<e>. The second quantifier C<.*> is free to gobble up the
+matching C<'e'>. The second quantifier C<.*> is free to gobble up the
rest of the string.
$x =~ /(m{1,2}?)(.*?)$/; # matches,
C<'m'> in C<programming>. At this position, the minimal C<m{1,2}?>
matches just one C<'m'>. Although the second quantifier C<.*?> would
prefer to match no characters, it is constrained by the end-of-string
-anchor C<$> to match the rest of the string.
+anchor C<'$'> to match the rest of the string.
$x =~ /(.*?)(m{1,2}?)(.*)$/; # matches,
# $1 = 'The progra'
# $3 = 'ming republic of Perl'
In this regexp, you might expect the first minimal quantifier C<.*?>
-to match the empty string, because it is not constrained by a C<^>
+to match the empty string, because it is not constrained by a C<'^'>
anchor to match the beginning of the word. Principle 0 applies here,
however. Because it is possible for the whole regexp to match at the
start of the string, it I<will> match at the start of the string. Thus
-the first quantifier has to match everything up to the first C<m>. The
-second minimal quantifier matches just one C<m> and the third
+the first quantifier has to match everything up to the first C<'m'>. The
+second minimal quantifier matches just one C<'m'> and the third
quantifier matches the rest of the string.
$x =~ /(.??)(m{1,2})(.*)$/; # matches,
=over 4
-=item 0
+=item Z<>0. Start with the first letter in the string C<'t'>.
-Start with the first letter in the string 't'.
+E<nbsp>
-=item 1
+=item Z<>1. The first quantifier C<'.*'> starts out by matching the whole
+string "C<the cat in the hat>".
-The first quantifier '.*' starts out by matching the whole
-string 'the cat in the hat'.
+E<nbsp>
-=item 2
+=item Z<>2. C<'a'> in the regexp element C<'at'> doesn't match the end
+of the string. Backtrack one character.
-'a' in the regexp element 'at' doesn't match the end of the
-string. Backtrack one character.
+E<nbsp>
-=item 3
+=item Z<>3. C<'a'> in the regexp element C<'at'> still doesn't match
+the last letter of the string C<'t'>, so backtrack one more character.
-'a' in the regexp element 'at' still doesn't match the last
-letter of the string 't', so backtrack one more character.
+E<nbsp>
-=item 4
+=item Z<>4. Now we can match the C<'a'> and the C<'t'>.
-Now we can match the 'a' and the 't'.
+E<nbsp>
-=item 5
-
-Move on to the third element '.*'. Since we are at the end of
-the string and '.*' can match 0 times, assign it the empty string.
+=item Z<>5. Move on to the third element C<'.*'>. Since we are at the
+end of the string and C<'.*'> can match 0 times, assign it the empty
+string.
-=item 6
+E<nbsp>
-We are done!
+=item Z<>6. We are done!
=back
/(a|b+)*/;
The problem is the nested indeterminate quantifiers. There are many
-different ways of partitioning a string of length n between the C<+>
-and C<*>: one repetition with C<b+> of length n, two repetitions with
+different ways of partitioning a string of length n between the C<'+'>
+and C<'*'>: one repetition with C<b+> of length n, two repetitions with
the first C<b+> length k and the second with length n-k, m repetitions
-whose bits add up to length n, etc. In fact there are an exponential
+whose bits add up to length n, I<etc>. In fact there are an exponential
number of ways to partition a string as a function of its length. A
regexp may get lucky and match early in the process, but if there is
no match, Perl will try I<every> possibility before giving up. So be
-careful with nested C<*>'s, C<{n,m}>'s, and C<+>'s. The book
+careful with nested C<'*'>'s, C<{n,m}>'s, and C<'+'>'s. The book
I<Mastering Regular Expressions> by Jeffrey Friedl gives a wonderful
discussion of this and other efficiency issues.
Whenever this is applied to a string which doesn't quite meet the
pattern's expectations such as S<C<"abc ">> or S<C<"abc def ">>,
-the regex engine will backtrack, approximately once for each character
+the regexp engine will backtrack, approximately once for each character
in the string. But we know that there is no way around taking I<all>
of the initial word characters to match the first repetition, that I<all>
spaces must be eaten by the middle part, and the same goes for the second
word.
With the introduction of the I<possessive quantifiers> in Perl 5.10, we
-have a way of instructing the regex engine not to backtrack, with the
-usual quantifiers with a C<+> appended to them. This makes them greedy as
+have a way of instructing the regexp engine not to backtrack, with the
+usual quantifiers with a C<'+'> appended to them. This makes them greedy as
well as stingy; once they succeed they won't give anything back to permit
another solution. They have the following meanings:
observation here is that I<both> integers and numbers with decimal
points are allowed in front of an exponent. Then exponents, like the
overall sign, are independent of whether we are matching numbers with
-or without decimal points, and can be 'decoupled' from the
+or without decimal points, and can be "decoupled" from the
mantissa. The overall form of the regexp now becomes clear:
/^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;
-The exponent is an C<e> or C<E>, followed by an integer. So the
+The exponent is an C<'e'> or C<'E'>, followed by an integer. So the
exponent regexp is
/[eE][+-]?\d+/; # exponent
/^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/; # Ta da!
Long regexps like this may impress your friends, but can be hard to
-decipher. In complex situations like this, the C<//x> modifier for a
+decipher. In complex situations like this, the C</x> modifier for a
match is invaluable. It allows one to put nearly arbitrary whitespace
and comments into a regexp without affecting their meaning. Using it,
-we can rewrite our 'extended' regexp in the more pleasing form
+we can rewrite our "extended" regexp in the more pleasing form
/^
[+-]? # first, match an optional sign
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
- ([eE][+-]?\d+)? # finally, optionally match an exponent
+ ( [eE] [+-]? \d+ )? # finally, optionally match an exponent
$/x;
If whitespace is mostly irrelevant, how does one include space
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
- ([eE][+-]?\d+)? # finally, optionally match an exponent
+ ( [eE] [+-]? \d+ )? # finally, optionally match an exponent
$/x;
In this form, it is easier to see a way to simplify the
)? # ? takes care of integers of the form a
|\.\d+ # mantissa of the form .b
)
- ([eE][+-]?\d+)? # finally, optionally match an exponent
+ ( [eE] [+-]? \d+ )? # finally, optionally match an exponent
$/x;
-or written in the compact form,
+Starting in Perl v5.26, specifying C</xx> changes the square-bracketed
+portions of a pattern to ignore tabs and space characters unless they
+are escaped by preceding them with a backslash. So, we could write
+
+ /^
+ [ + - ]?\ * # first, match an optional sign
+ ( # then match integers or f.p. mantissas:
+ \d+ # start out with a ...
+ (
+ \.\d* # mantissa of the form a.b or a.
+ )? # ? takes care of integers of the form a
+ |\.\d+ # mantissa of the form .b
+ )
+ ( [ e E ] [ + - ]? \d+ )? # finally, optionally match an exponent
+ $/xx;
+
+This doesn't really improve the legibility of this example, but it's
+available in case you want it. Squashing the pattern down to the
+compact form, we have
/^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;
C</regexp/> and arbitrary delimiter C<m!regexp!> forms. We have used
the binding operator C<=~> and its negation C<!~> to test for string
matches. Associated with the matching operator, we have discussed the
-single line C<//s>, multi-line C<//m>, case-insensitive C<//i> and
-extended C<//x> modifiers. There are a few more things you might
+single line C</s>, multi-line C</m>, case-insensitive C</i> and
+extended C</x> modifiers. There are a few more things you might
want to know about matching operators.
=head3 Prohibiting substitution
}
Similar to strings, C<m''> acts like apostrophes on a regexp; all other
-C<m> delimiters act like quotes. If the regexp evaluates to the empty string,
+C<'m'> delimiters act like quotes. If the regexp evaluates to the empty string,
the regexp in the I<last successful match> is used instead. So we have
"dog" =~ /d/; # 'd' matches
=head3 Global matching
-The final two modifiers we will disccuss here,
-C<//g> and C<//c>, concern multiple matches.
-The modifier C<//g> stands for global matching and allows the
+The final two modifiers we will discuss here,
+C</g> and C</c>, concern multiple matches.
+The modifier C</g> stands for global matching and allows the
matching operator to match within a string as many times as possible.
In scalar context, successive invocations against a string will have
-C<//g> jump from match to match, keeping track of position in the
+C</g> jump from match to match, keeping track of position in the
string as it goes along. You can get or set the position with the
C<pos()> function.
-The use of C<//g> is shown in the following example. Suppose we have
+The use of C</g> is shown in the following example. Suppose we have
a string that consists of words separated by spaces. If we know how
many words there are in advance, we could extract the words using
groupings:
# $3 = 'house'
But what if we had an indeterminate number of words? This is the sort
-of task C<//g> was made for. To extract all words, form the simple
+of task C</g> was made for. To extract all words, form the simple
regexp C<(\w+)> and loop over all matches with C</(\w+)/g>:
while ($x =~ /(\w+)/g) {
A failed match or changing the target string resets the position. If
you don't want the position reset after failure to match, add the
-C<//c>, as in C</regexp/gc>. The current position in the string is
+C</c>, as in C</regexp/gc>. The current position in the string is
associated with the string, not the regexp. This means that different
strings have different positions and their respective positions can be
set or read independently.
-In list context, C<//g> returns a list of matched groupings, or if
+In list context, C</g> returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regexp. So if
we wanted just the words, we could use
@words = ($x =~ /(\w+)/g); # matches,
- # $word[0] = 'cat'
- # $word[1] = 'dog'
- # $word[2] = 'house'
+ # $words[0] = 'cat'
+ # $words[1] = 'dog'
+ # $words[2] = 'house'
-Closely associated with the C<//g> modifier is the C<\G> anchor. The
-C<\G> anchor matches at the point where the previous C<//g> match left
+Closely associated with the C</g> modifier is the C<\G> anchor. The
+C<\G> anchor matches at the point where the previous C</g> match left
off. C<\G> allows us to easily do context-sensitive matching:
$metric = 1; # use metric units
}
$x =~ /\G\s+(widget|sprocket)/g; # continue processing
-The combination of C<//g> and C<\G> allows us to process the string a
+The combination of C</g> and C<\G> allows us to process the string a
bit at a time and use arbitrary Perl logic to decide what to do next.
Currently, the C<\G> anchor is only fully supported when used to anchor
to the start of the pattern.
$dna =~ /TGA/;
doesn't work; it may match a C<TGA>, but there is no guarantee that
-the match is aligned with codon boundaries, e.g., the substring
+the match is aligned with codon boundaries, I<e.g.>, the substring
S<C<GTT GAA>> gives a match. A better solution is
while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *?
desired.
(There are other regexp modifiers that are available, such as
-C<//o>, C<//d>, and C<//l>, but their specialized uses are beyond the
+C</o>, but their specialized uses are beyond the
scope of this introduction. )
=head3 Search and replace
C<s///> operator. The general form is
C<s/regexp/replacement/modifiers>, with everything we know about
regexps and modifiers applying in this case as well. The
-C<replacement> is a Perl double-quoted string that replaces in the
+I<replacement> is a Perl double-quoted string that replaces in the
string whatever is matched with the C<regexp>. The operator C<=~> is
also used here to associate a string with C<s///>. If matching
against C<$_>, the S<C<$_ =~>> can be dropped. If there is a match,
In the last example, the whole string was matched, but only the part
inside the single quotes was grouped. With the C<s///> operator, the
-matched variables C<$1>, C<$2>, etc. are immediately available for use
+matched variables C<$1>, C<$2>, I<etc>. are immediately available for use
in the replacement expression, so we use C<$1> to replace the quoted
string with just what was quoted. With the global modifier, C<s///g>
will search and replace all occurrences of the regexp in the string:
$x =~ s/4/four/g; # does it all:
# $x contains "I batted four for four"
-If you prefer 'regex' over 'regexp' in this tutorial, you could use
+If you prefer "regex" over "regexp" in this tutorial, you could use
the following program to replace it:
% cat > simple_replace
substitutions:
$x = "Cats are great.";
- print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
+ print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~
+ s/Frogs/Hedgehogs/r, "\n";
# prints "Hedgehogs are great."
A modifier available specifically to search and replace is the
character frequencies in a line:
$x = "Bill the cat";
- $x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself
+ $x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself
print "frequency of '$_' is $chars{$_}\n"
foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);
used C<s'''>, then the regexp and replacement are
treated as single-quoted strings and there are no
variable substitutions. C<s///> in list context
-returns the same thing as in scalar context, i.e., the number of
+returns the same thing as in scalar context, I<i.e.>, the number of
matches.
=head3 The split function
# $parts[5] = '/'
# $parts[6] = 'perl'
-Since the first character of $x matched the regexp, C<split> prepended
+Since the first character of C<$x> matched the regexp, C<split> prepended
an empty initial element to the list.
If you have read this far, congratulations! You now have all the basic
$x = "\QThat !^*&%~& cat!";
$x =~ /\Q!^*&%~&\E/; # check for rough language
-It does not protect C<$> or C<@>, so that variables can still be
+It does not protect C<'$'> or C<'@'>, so that variables can still be
substituted.
C<\Q>, C<\L>, C<\l>, C<\U>, C<\u> and C<\E> are actually part of
double-quotish syntax, and not part of regexp syntax proper. They will
-work if they appear in a regular expression embeddded directly in a
+work if they appear in a regular expression embedded directly in a
program, but not when contained in a string that is interpolated in a
pattern.
-With the advent of 5.6.0, Perl regexps can handle more than just the
-standard ASCII character set. Perl now supports I<Unicode>, a standard
+Perl regexps can handle more than just the
+standard ASCII character set. Perl supports I<Unicode>, a standard
for representing the alphabets from virtually all of the world's written
languages, and a host of symbols. Perl's text strings are Unicode strings, so
they can contain characters with a value (codepoint or character number) higher
a matching operation will treat the string to be searched as a sequence
of characters, not bytes. The answer to 1) is that Unicode characters
greater than C<chr(255)> are represented using the C<\x{hex}> notation, because
-\x hex (without curly braces) doesn't go further than 255. (Starting in Perl
-5.14, if you're an octal fan, you can also use C<\o{oct}>.)
+C<\x>I<XY> (without curly braces and I<XY> are two hex digits) doesn't
+go further than 255. (Starting in Perl 5.14, if you're an octal fan,
+you can also use C<\o{oct}>.)
/\x{263a}/; # match a Unicode smiley face :)
represent or match the astrological sign for the planet Mercury, we
could use
- use charnames ":full"; # use named chars with Unicode full names
$x = "abc\N{MERCURY}def";
$x =~ /\N{MERCURY}/; # matches
-One can also use short names or restrict names to a certain alphabet:
+One can also use "short" names:
- use charnames ':full';
print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";
-
- use charnames ":short";
print "\N{greek:Sigma} is an upper-case sigma.\n";
+You can also restrict names to a certain alphabet by specifying the
+L<charnames> pragma:
+
use charnames qw(greek);
print "\N{sigma} is Greek sigma\n";
-A list of full names can be found in F<NamesList.txt> in the Unicode standard
-(available at L<http://www.unicode.org/Public/UNIDATA/>).
-
-The answer to requirement 2), as of 5.6.0, is that a regexp (mostly)
-uses Unicode characters. (For messy backward compatibility reasons,
-most but not all semantics of a match will assume Unicode, unless,
-starting in Perl 5.14, you tell it to use full Unicode. You can do this
-explicitly by using the C<//u> modifier, or you can ask Perl to use the
-modifier implicitly for all regexes in a scope by using C<use 5.012> (or
-higher) or C<use feature 'unicode_strings'>.) If you want to handle
-Unicode properly, you should ensure that one of these is the case.)
+An index of character names is available on-line from the Unicode
+Consortium, L<http://www.unicode.org/charts/charindex.html>; explanatory
+material with links to other resources at
+L<http://www.unicode.org/standard/where>.
+
+The answer to requirement 2) is that a regexp (mostly)
+uses Unicode characters. The "mostly" is for messy backward
+compatibility reasons, but starting in Perl 5.14, any regexp compiled in
+the scope of a C<use feature 'unicode_strings'> (which is automatically
+turned on within the scope of a C<use 5.012> or higher) will turn that
+"mostly" into "always". If you want to handle Unicode properly, you
+should ensure that C<'unicode_strings'> is turned on.
Internally, this is encoded to bytes using either UTF-8 or a native 8
bit encoding, depending on the history of the string, but conceptually
it is a sequence of characters, not bytes. See L<perlunitut> for a
tutorial about that.
-Let us now discuss Unicode character classes. Just as with Unicode
-characters, there are named Unicode character classes represented by the
-C<\p{name}> escape sequence. Closely associated is the C<\P{name}>
-character class, which is the negation of the C<\p{name}> class. For
-example, to match lower and uppercase characters,
+Let us now discuss Unicode character classes, most usually called
+"character properties". These are represented by the C<\p{I<name>}>
+escape sequence. The negation of this is C<\P{I<name>}>. For example,
+to match lower and uppercase characters,
- use charnames ":full"; # use named chars with Unicode full names
$x = "BOB";
$x =~ /^\p{IsUpper}/; # matches, uppercase char class
$x =~ /^\P{IsUpper}/; # doesn't match, char class sans uppercase
$x =~ /^\p{IsLower}/; # doesn't match, lowercase char class
$x =~ /^\P{IsLower}/; # matches, char class sans lowercase
-(The "Is" is optional.)
-
-Here is the association between some Perl named classes and the
-traditional Unicode classes:
-
- Perl class name Unicode class name or regular expression
-
- IsAlpha /^[LM]/
- IsAlnum /^[LMN]/
- IsASCII $code <= 127
- IsCntrl /^C/
- IsBlank $code =~ /^(0020|0009)$/ || /^Z[^lp]/
- IsDigit Nd
- IsGraph /^([LMNPS]|Co)/
- IsLower Ll
- IsPrint /^([LMNPS]|Co|Zs)/
- IsPunct /^P/
- IsSpace /^Z/ || ($code =~ /^(0009|000A|000B|000C|000D)$/
- IsSpacePerl /^Z/ || ($code =~ /^(0009|000A|000C|000D|0085|2028|2029)$/
- IsUpper /^L[ut]/
- IsWord /^[LMN]/ || $code eq "005F"
- IsXDigit $code =~ /^00(3[0-9]|[46][1-6])$/
-
-You can also use the official Unicode class names with C<\p> and
-C<\P>, like C<\p{L}> for Unicode 'letters', C<\p{Lu}> for uppercase
-letters, or C<\P{Nd}> for non-digits. If a C<name> is just one
-letter, the braces can be dropped. For instance, C<\pM> is the
-character class of Unicode 'marks', for example accent marks.
-For the full list see L<perlunicode>.
-
-Unicode has also been separated into various sets of characters
-which you can test with C<\p{...}> (in) and C<\P{...}> (not in).
-To test whether a character is (or is not) an element of a script
-you would use the script name, for example C<\p{Latin}>, C<\p{Greek}>,
-or C<\P{Katakana}>.
+(The "C<Is>" is optional.)
+
+There are many, many Unicode character properties. For the full list
+see L<perluniprops>. Most of them have synonyms with shorter names,
+also listed there. Some synonyms are a single character. For these,
+you can drop the braces. For instance, C<\pM> is the same thing as
+C<\p{Mark}>, meaning things like accent marks.
+
+The Unicode C<\p{Script}> and C<\p{Script_Extensions}> properties are
+used to categorize every Unicode character into the language script it
+is written in. (C<Script_Extensions> is an improved version of
+C<Script>, which is retained for backward compatibility, and so you
+should generally use C<Script_Extensions>.)
+For example,
+English, French, and a bunch of other European languages are written in
+the Latin script. But there is also the Greek script, the Thai script,
+the Katakana script, I<etc>. You can test whether a character is in a
+particular script (based on C<Script_Extensions>) with, for example
+C<\p{Latin}>, C<\p{Greek}>, or C<\p{Katakana}>. To test if it isn't in
+the Balinese script, you would use C<\P{Balinese}>.
What we have described so far is the single form of the C<\p{...}> character
classes. There is also a compound form which you may run into. These
-look like C<\p{name=value}> or C<\p{name:value}> (the equals sign and colon
+look like C<\p{I<name>=I<value>}> or C<\p{I<name>:I<value>}> (the equals sign and colon
can be used interchangeably). These are more general than the single form,
and in fact most of the single forms are just Perl-defined shortcuts for common
compound forms. For example, the script examples in the previous paragraph
-could be written equivalently as C<\p{Script=Latin}>, C<\p{Script:Greek}>, and
-C<\P{script=katakana}> (case is irrelevant between the C<{}> braces). You may
+could be written equivalently as C<\p{Script_Extensions=Latin}>, C<\p{Script_Extensions:Greek}>,
+C<\p{script_extensions=katakana}>, and C<\P{script_extensions=balinese}> (case is irrelevant
+between the C<{}> braces). You may
never have to use the compound forms, but sometimes it is necessary, and their
use can make your code easier to understand.
C<\X> is an abbreviation for a character class that comprises
a Unicode I<extended grapheme cluster>. This represents a "logical character":
what appears to be a single character, but may be represented internally by more
-than one. As an example, using the Unicode full names,
e.g., S<C<A + COMBINING
-RING>> is a grapheme cluster with base character C<A> and combining character
-S<C<COMBINING RING>>, which translates in Danish to A with the circle atop it,
-as in the word Angstrom.
+than one. As an example, using the Unicode full names,
I<e.g.>, "S<A + COMBINING
+RING>" is a grapheme cluster with base character "A" and combining character
+"S<COMBINING RING>, which translates in Danish to "A" with the circle atop it,
+as in the word E<Aring>ngstrom.
For the full and latest information about Unicode see the latest
Unicode standard, or the Unicode Consortium's website L<http://www.unicode.org>
As if all those classes weren't enough, Perl also defines POSIX-style
-character classes. These have the form C<[:name:]>, with C<name> the
+character classes. These have the form C<[:I<name>:]>, with I<name> the
name of the POSIX class. The POSIX classes are C<alpha>, C<alnum>,
C<ascii>, C<cntrl>, C<digit>, C<graph>, C<lower>, C<print>, C<punct>,
C<space>, C<upper>, and C<xdigit>, and two extensions, C<word> (a Perl
-extension to match C<\w>), and C<blank> (a GNU extension). The C<//a>
+extension to match C<\w>), and C<blank> (a GNU extension). The C</a>
modifier restricts these to matching just in the ASCII range; otherwise
they can match the same as their corresponding Perl Unicode classes:
-C<[:upper:]> is the same as C<\p{IsUpper}>, etc. (There are some
+C<[:upper:]> is the same as C<\p{IsUpper}>, I<etc>. (There are some
exceptions and gotchas with this; see L<perlrecharclass> for a full
discussion.) The C<[:digit:]>, C<[:word:]>, and
C<[:space:]> correspond to the familiar C<\d>, C<\w>, and C<\s>
-character classes. To negate a POSIX class, put a C<^> in front of
-the name, so that, e.g., C<[:^digit:]> corresponds to C<\D> and, under
+character classes. To negate a POSIX class, put a C<'^'> in front of
+the name, so that, I<e.g.>, C<[:^digit:]> corresponds to C<\D> and, under
Unicode, C<\P{IsDigit}>. The Unicode and POSIX character classes can
be used just like C<\d>, with the exception that POSIX character
classes can only be used inside of a character class:
$x =~ /(abc)?$reg/; # still matches
As with the matching operator, the regexp quote can use different
-delimiters, e.g., C<qr!!>, C<qr{}> or C<qr~~>. Apostrophes
+delimiters, I<e.g.>, C<qr!!>, C<qr{}> or C<qr~~>. Apostrophes
as delimiters (C<qr''>) inhibit any interpolation.
Pre-compiled regexps are useful for creating dynamic matches that
% cat > keymatch
#!/usr/bin/perl
$kwds = 'copy compare list print';
- while( $command = <> ){
- $command =~ s/^\s+|\s+$//g; # trim leading and trailing spaces
- if( ( @matches = $kwds =~ /\b$command\w*/g ) == 1 ){
+ while( $cmd = <> ){
+ $cmd =~ s/^\s+|\s+$//g; # trim leading and trailing spaces
+ if( ( @matches = $kwds =~ /\b$cmd\w*/g ) == 1 ){
print "command: '@matches'\n";
} elsif( @matches == 0 ){
- print "no such command: '$command'\n";
+ print "no such command: '$cmd'\n";
} else {
- print "not unique: '$command' (could be one of: @matches)\n";
+ print "not unique: '$cmd' (could be one of: @matches)\n";
}
}
^D
Rather than trying to match the input against the keywords, we match the
combined set of keywords against the input. The pattern matching
-operation S<C<$kwds =~ /\b($command\w*)/g>> does several things at the
+operation S<C<$kwds =~ /\b($cmd\w*)/g>> does several things at the
same time. It makes sure that the given command begins where a keyword
begins (C<\b>). It tolerates abbreviations due to the added C<\w*>. It
tells us the number of matches (C<scalar @matches>) and all the keywords
/(?# Match an integer:)[+-]?\d+/;
This style of commenting has been largely superseded by the raw,
-freeform commenting that is allowed with the C<//x> modifier.
+freeform commenting that is allowed with the C</x> modifier.
-Most modifiers, such as C<//i>, C<//m>, C<//s> and C<//x> (or any
+Most modifiers, such as C</i>, C</m>, C</s> and C</x> (or any
combination thereof) can also be embedded in
a regexp using C<(?i)>, C<(?m)>, C<(?s)>, and C<(?x)>. For instance,
/x;
Embedded modifiers can have two important advantages over the usual
-modifiers. Embedded modifiers allow a custom set of modifiers to
+modifiers. Embedded modifiers allow a custom set of modifiers for
I<each> regexp pattern. This is great for matching an array of regexps
that must have different modifiers:
}
}
-The second advantage is that embedded modifiers (except C<//p>, which
+The second advantage is that embedded modifiers (except C</p>, which
modifies the entire regexp) only affect the regexp
inside the group the embedded modifier is contained in. So grouping
can be used to localize the modifier's effects:
/Answer: ((?i)yes)/; # matches 'Answer: yes', 'Answer: YES', etc.
Embedded modifiers can also turn off any modifiers already present
-by using, e.g., C<(?-i)>. Modifiers can also be combined into
-a single expression, e.g., C<(?s-i)> turns on single line mode and
+by using, I<e.g.>, C<(?-i)>. Modifiers can also be combined into
+a single expression, I<e.g.>, C<(?s-i)> turns on single line mode and
turns off case insensitivity.
Embedded modifiers may also be added to a non-capturing grouping.
This section concerns the lookahead and lookbehind assertions. First,
a little background.
-In Perl regular expressions, most regexp elements 'eat up' a certain
+In Perl regular expressions, most regexp elements "eat up" a certain
amount of string when they match. For instance, the regexp element
-C<[abc}]> eats up one character of the string when it matches, in the
+C<[abc]> eats up one character of the string when it matches, in the
sense that Perl moves to the next character position in the string
after the match. There are some elements, however, that don't eat up
characters (advance the character position) if they match. The examples
-we have seen so far are the anchors. The anchor C<^> matches the
+we have seen so far are the anchors. The anchor C<'^'> matches the
beginning of the line, but doesn't eat any characters. Similarly, the
word boundary anchor C<\b> matches wherever a character matching C<\w>
is next to a character that doesn't, but it doesn't eat up any
doesn't satisfy us, we must backtrack.
Checking the environment entails either looking ahead on the trail,
-looking behind, or both. C<^> looks behind, to see that there are no
-characters before. C<$> looks ahead, to see that there are no
+looking behind, or both. C<'^'> looks behind, to see that there are no
+characters before. C<'$'> looks ahead, to see that there are no
characters after. C<\b> looks both ahead and behind, to see if the
characters on either side differ in their "word-ness".
second regexp, the substrings captured are those of the whole regexp
itself. Lookahead C<(?=regexp)> can match arbitrary regexps, but
lookbehind C<< (?<=fixed-regexp) >> only works for regexps of fixed
-width, i.e., a fixed number of characters long. Thus
+width, I<i.e.>, a fixed number of characters long. Thus
C<< (?<=(ab|bc)) >> is fine, but C<< (?<=(ab)*) >> is not. The
negated versions of the lookahead and lookbehind assertions are
denoted by C<(?!regexp)> and C<< (?<!fixed-regexp) >> respectively.
$x =~ /foo(?!baz)/; # matches, 'baz' doesn't follow 'foo'
$x =~ /(?<!\s)foo/; # matches, there is no \s before 'foo'
-The C<\C> is unsupported in lookbehind, because the already
-treacherous definition of C<\C> would become even more so
-when going backwards.
-
Here is an example where a string containing blank-separated words,
numbers and single dashes is to be split into its components.
Using C</\s+/> alone won't work, because spaces are not required between
$x =~ /a*ab/; # matches
This obviously matches, but in the process of matching, the
-subexpression C<a*> first grabbed the C<a>. Doing so, however,
+subexpression C<a*> first grabbed the C<'a'>. Doing so, however,
wouldn't allow the whole regexp to match, so after backtracking, C<a*>
-eventually gave back the C<a> and matched the empty string. Here, what
+eventually gave back the C<'a'> and matched the empty string. Here, what
C<a*> matched was I<dependent> on what the rest of the regexp matched.
Contrast that with an independent subexpression:
$x =~ /(?>a*)ab/; # doesn't match!
The independent subexpression C<< (?>a*) >> doesn't care about the rest
-of the regexp, so it sees an C<a> and grabs it. Then the rest of the
+of the regexp, so it sees an C<'a'> and grabs it. Then the rest of the
regexp C<ab> cannot match. Because C<< (?>a*) >> is independent, there
is no backtracking and the independent subexpression does not give
-up its C<a>. Thus the match of the regexp as a whole fails. A similar
+up its C<'a'>. Thus the match of the regexp as a whole fails. A similar
behavior occurs with completely independent regexps:
$x = "ab";
$x =~ /a*/g; # matches, eats an 'a'
$x =~ /\Gab/g; # doesn't match, no 'a' available
-Here C<//g> and C<\G> create a 'tag team' handoff of the string from
+Here C</g> and C<\G> create a "tag team" handoff of the string from
one regexp to the other. Regexps with an independent subexpression are
much like this, with a handoff of the string to the independent
subexpression, and a handoff of the string back to the enclosing
regexp matches:
$x = "abc(de(fg)h"; # unbalanced parentheses
- $x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x;
+ $x =~ /\( ( [ ^ () ]+ | \( [ ^ () ]* \) )+ \)/xx;
The regexp matches an open parenthesis, one or more copies of an
alternation, and a close parenthesis. The alternation is two-way, with
prevent useless backtracking at some point. This can be done by
enclosing the inner quantifier as an independent subexpression:
- $x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x;
+ $x =~ /\( ( (?> [ ^ () ]+ ) | \([ ^ () ]* \) )+ \)/xx;
Here, C<< (?>[^()]+) >> breaks the degeneracy of string partitioning
by gobbling up as much of the string as possible and keeping it. Then
A I<conditional expression> is a form of if-then-else statement
that allows one to choose which patterns are to be matched, based on
some condition. There are two types of conditional expression:
-C<(?(condition)yes-regexp)> and
-C<(?(condition)yes-regexp|no-regexp)>. C<(?(condition)yes-regexp)> is
-like an S<C<'if () {}'>> statement in Perl. If the C<condition> is true,
-the C<yes-regexp> will be matched. If the C<condition> is false, the
-C<yes-regexp> will be skipped and Perl will move onto the next regexp
+C<(?(I<condition>)I<yes-regexp>)> and
+C<(?(condition)I<yes-regexp>|I<no-regexp>)>.
+C<(?(I<condition>)I<yes-regexp>)> is
+like an S<C<'if () {}'>> statement in Perl. If the I<condition> is true,
+the I<yes-regexp> will be matched. If the I<condition> is false, the
+I<yes-regexp> will be skipped and Perl will move onto the next regexp
element. The second form is like an S<C<'if () {} else {}'>> statement
-in Perl. If the C<condition> is true, the C<yes-regexp> will be
-matched, otherwise the C<no-regexp> will be matched.
+in Perl. If the I<condition> is true, the I<yes-regexp> will be
+matched, otherwise the I<no-regexp> will be matched.
-The C<condition> can have several forms. The first form is simply an
-integer in parentheses C<(integer)>. It is true if the corresponding
-backreference C<\integer> matched earlier in the regexp. The same
+The I<condition> can have several forms. The first form is simply an
+integer in parentheses C<(I<integer>)>. It is true if the corresponding
+backreference C<\I<integer>> matched earlier in the regexp. The same
thing can be done with a name associated with a capture group, written
-as C<< (<name>) >> or C<< ('name') >>. The second form is a bare
+as C<<< (E<lt>I<name>E<gt>) >>> or C<< ('I<name>') >>. The second form is a bare
zero-width assertion C<(?...)>, either a lookahead, a lookbehind, or a
code assertion (discussed in the next section). The third set of forms
provides tests that return true if the expression is executed within
a recursion (C<(R)>) or is being called from some capturing group,
referenced either by number (C<(R1)>, C<(R2)>,...) or by name
-(C<(R&name)>).
+(C<(R&I<name>)>).
The integer or name form of the C<condition> allows us to choose,
with more flexibility, what to match based on what matched earlier in the
/[ATGC]+(?(?<=AA)G|C)$/;
matches a DNA sequence such that it either ends in C<AAG>, or some
-other base pair combination and C<C>. Note that the form is
+other base pair combination and C<'C'>. Note that the form is
C<< (?(?<=AA)G|C) >> and not C<< (?((?<=AA))G|C) >>; for the
lookahead, lookbehind or code assertions, the parentheses around the
conditional are not needed.
Starting with Perl 5.10, it is possible to define named subpatterns in
a section of the pattern so that they can be called up by name
anywhere in the pattern. This syntactic pattern for this definition
-group is C<< (?(DEFINE)(?<name>pattern)...) >>. An insertion
-of a named pattern is written as C<(?&name)>.
+group is C<< (?(DEFINE)(?<I<name>>I<pattern>)...) >>. An insertion
+of a named pattern is written as C<(?&I<name>)>.
The example below illustrates this feature using the pattern for
floating point numbers that was presented earlier on. The three
subpatterns that are used more than once are the optional sign, the
-digit sequence for an integer and the decimal fraction. The DEFINE
+digit sequence for an integer and the decimal fraction. The C<DEFINE>
group at the end of the pattern contains their definition. Notice
that the decimal fraction pattern is the first place where we can
reuse the integer pattern.
This feature (introduced in Perl 5.10) significantly extends the
power of Perl's pattern matching. By referring to some other
capture group anywhere in the pattern with the construct
-C<(?
group-ref)>, the I<pattern> within the referenced group is used
+C<(?
I<group-ref>)>, the I<pattern> within the referenced group is used
as an independent subpattern in place of the group reference itself.
Because the group reference may be contained I<within> the group it
refers to, it is now possible to apply pattern matching to tasks that
In C<(?...)> both absolute and relative backreferences may be used.
The entire pattern can be reinserted with C<(?R)> or C<(?0)>.
-If you prefer to name your groups, you can use C<(?&name)> to
+If you prefer to name your groups, you can use C<(?&I<name>)> to
recurse into that group.
Normally, regexps are a part of Perl expressions.
I<Code evaluation> expressions turn that around by allowing
arbitrary Perl code to be a part of a regexp. A code evaluation
-expression is denoted C<(?{code})>, with I<code> a string of Perl
+expression is denoted C<(?{I<code>})>, with I<code> a string of Perl
statements.
-Be warned that this feature is considered experimental, and may be
-changed without notice.
-
Code expressions are zero-width assertions, and the value they return
depends on their environment. There are two possibilities: either the
code expression is used as a conditional in a conditional expression
-C<(?(condition)...)>, or it is not. If the code expression is a
-conditional, the code is evaluated and the result (i.e., the result of
+C<(?(I<condition>)...)>, or it is not. If the code expression is a
+conditional, the code is evaluated and the result (I<i.e.>, the result of
the last statement) is used to determine truth or falsehood. If the
code expression is not used as a conditional, the assertion always
evaluates true and the result is put into the special variable
Hmm. What happened here? If you've been following along, you know that
the above pattern should be effectively (almost) the same as the last one;
-enclosing the C<d> in a character class isn't going to change what it
+enclosing the C<'d'> in a character class isn't going to change what it
matches. So why does the first not print while the second one does?
-The answer lies in the optimizations the regex engine makes. In the first
+The answer lies in the optimizations the regexp engine makes. In the first
case, all the engine sees are plain old characters (aside from the
-C<?{}> construct). It's smart enough to realize that the string 'ddd'
+C<?{}> construct). It's smart enough to realize that the string C<'ddd'>
doesn't occur in our target string before actually running the pattern
through. But in the second case, we've tricked it into thinking that our
pattern is more complicated. It takes a look, sees our
have a match.
To take a closer look at how the engine does optimizations, see the
-section L<"Pragmas and debugging"> below.
+section L</"Pragmas and debugging"> below.
More fun with C<?{}>:
backtracks over a code expression and if the variables used within are
localized using C<local>, the changes in the variables produced by the
code expression are undone! Thus, if we wanted to count how many times
-a character got matched inside a group, we could use, e.g.,
+a character got matched inside a group, we could use, I<e.g.>,
$x = "aaaa";
$count = 0; # initialize 'a' count
properly in the presence of backtracking.
This example uses a code expression in a conditional to match a
-definite article, either 'the' in English or 'der|die|das' in German:
+definite article, either C<'the'> in English or C<'der|die|das'> in
+German:
$lang = 'DE'; # use German
...
)
/xi;
-Note that the syntax here is C<(?(?{...})yes-regexp|no-regexp)>, not
-C<(?((?{...}))yes-regexp|no-regexp)>. In other words, in the case of a
+Note that the syntax here is C<(?(?{...})I<yes-regexp>|I<no-regexp>)>, not
+C<(?((?{...}))I<yes-regexp>|I<no-regexp>)>. In other words, in the case of a
code expression, we don't need the extra parentheses around the
conditional.
-If you try to use code expressions with interpolating variables, Perl
-may surprise you:
+If you try to use code expressions where the code text is contained within
+an interpolated variable, rather than appearing literally in the pattern,
+Perl may surprise you:
$bar = 5;
$pat = '(?{ 1 })';
/foo(?{ $bar })bar/; # compiles ok, $bar not interpolated
- /foo(?{ 1 })$bar/; # compile error!
+ /foo(?{ 1 })$bar/; # compiles ok, $bar interpolated
/foo${pat}bar/; # compile error!
$pat = qr/(?{ $foo = 1 })/; # precompile code regexp
/foo${pat}bar/; # compiles ok
-If a regexp has (1) code expressions and interpolating variables, or
-(2) a variable that interpolates a code expression, Perl treats the
-regexp as an error. If the code expression is precompiled into a
-variable, however, interpolating is ok. The question is, why is this
-an error?
+If a regexp has a variable that interpolates a code expression, Perl
+treats the regexp as an error. If the code expression is precompiled into
+a variable, however, interpolating is ok. The question is, why is this an
+error?
The reason is that variable interpolation and code expressions
together pose a security risk. The combination is dangerous because
use re 'eval'; # throw caution out the door
$bar = 5;
$pat = '(?{ 1 })';
- /foo(?{ 1 })$bar/; # compiles ok
/foo${pat}bar/; # compiles ok
Another form of code expression is the I<pattern code expression>.
This final example contains both ordinary and pattern code
expressions. It detects whether a binary string C<1101010010001...> has a
-Fibonacci spacing 0,1,1,2,3,5,... of the C<1>'s:
+Fibonacci spacing 0,1,1,2,3,5,... of the C<'1'>'s:
$x = "1101010010001000001";
$z0 = ''; $z1 = '0'; # initial conditions
Note that the variables C<$z0> and C<$z1> are not substituted when the
regexp is compiled, as happens for ordinary variables outside a code
-expression. Rather, the code expressions are evaluated when Perl
-encounters them during the search for a match.
+expression. Rather, the whole code block is parsed as perl code at the
+same time as perl is compiling the code containing the literal regexp
+pattern.
-The regexp without the C<//x> modifier is
+This regexp without the C</x> modifier is
/^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/
Perl 5.10 introduced a number of control verbs intended to provide
detailed control over the backtracking process, by directly influencing
-the regexp engine and by providing monitoring techniques. As all
-the features in this group are experimental and subject to change or
-removal in a future version of Perl, the interested reader is
-referred to L<perlre/"Special Backtracking Control Verbs"> for a
-detailed description.
+the regexp engine and by providing monitoring techniques. See
+L<perlre/"Special Backtracking Control Verbs"> for a detailed
+description.
Below is just one example, illustrating the control verb C<(*FAIL)>,
which may be abbreviated as C<(*F)>. If this is inserted in a regexp
combination with embedded code.
%count = ();
- "supercalifragilisticexpialidoceous" =~
+ "supercalifragilisticexpialidocious" =~
/([aeiou])(?{ $count{$1}++; })(*FAIL)/i;
printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);
regexp engine proceeds until the entire string has been inspected.
(It's remarkable that an alternative solution using something like
- $count{lc($_)}++ for split('', "supercalifragilisticexpialidoceous");
+ $count{lc($_)}++ for split('', "supercalifragilisticexpialidocious");
printf "%3d '%s'\n", $count2{$_}, $_ for ( qw{ a e i o u } );
is considerably slower.)
termcap color sequences. Here is example output:
% perl -e 'use re "debug"; "abc" =~ /a*b+c/;'
- Compiling REx `a*b+c'
+ Compiling REx 'a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
5: EXACT <b>(0)
7: EXACT <c>(9)
9: END(0)
- floating `bc' at 0..2147483647 (checking floating) minlen 2
- Guessing start of match, REx `a*b+c' against `abc'...
- Found floating substr `bc' at offset 1...
+ floating 'bc' at 0..2147483647 (checking floating) minlen 2
+ Guessing start of match, REx 'a*b+c' against 'abc'...
+ Found floating substr 'bc' at offset 1...
Guessed: match at offset 0
- Matching REx `a*b+c' against `abc'
+ Matching REx 'a*b+c' against 'abc'
Setting an EVAL scope, savestack=3
- 0 <> <abc> | 1: STAR
-
EXACT <a> can match 1 times out of 32767...
+ 0 <> <abc> | 1: STAR
+ EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
- 1 <a> <bc> | 4: PLUS
- EXACT <b> can match 1 times out of 32767...
+ 1 <a> <bc> | 4: PLUS
+ EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
- 2 <ab> <c> | 7: EXACT <c>
- 3 <abc> <> | 9: END
+ 2 <ab> <c> | 7: EXACT <c>
+ 3 <abc> <> | 9: END
Match successful!
- Freeing REx: `a*b+c'
+ Freeing REx: 'a*b+c'
If you have gotten this far into the tutorial, you can probably guess
what the different parts of the debugging output tell you. The first
part
- Compiling REx `a*b+c'
+ Compiling REx 'a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
describes the compilation stage. C<STAR(4)> means that there is a
starred object, in this case C<'a'>, and if it matches, goto line 4,
-
i.e., C<PLUS(7)>. The middle lines describe some heuristics and
+
I<i.e.>, C<PLUS(7)>. The middle lines describe some heuristics and
optimizations performed before a match:
- floating `bc' at 0..2147483647 (checking floating) minlen 2
- Guessing start of match, REx `a*b+c' against `abc'...
- Found floating substr `bc' at offset 1...
+ floating 'bc' at 0..2147483647 (checking floating) minlen 2
+ Guessing start of match, REx 'a*b+c' against 'abc'...
+ Found floating substr 'bc' at offset 1...
Guessed: match at offset 0
Then the match is executed and the remaining lines describe the
process:
- Matching REx `a*b+c' against `abc'
+ Matching REx 'a*b+c' against 'abc'
Setting an EVAL scope, savestack=3
- 0 <> <abc> | 1: STAR
-
EXACT <a> can match 1 times out of 32767...
+ 0 <> <abc> | 1: STAR
+ EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
- 1 <a> <bc> | 4: PLUS
- EXACT <b> can match 1 times out of 32767...
+ 1 <a> <bc> | 4: PLUS
+ EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
- 2 <ab> <c> | 7: EXACT <c>
- 3 <abc> <> | 9: END
+ 2 <ab> <c> | 7: EXACT <c>
+ 3 <abc> <> | 9: END
Match successful!
- Freeing REx: `a*b+c'
+ Freeing REx: 'a*b+c'
Each step is of the form S<C<< n <x> <y> >>>, with C<< <x> >> the
part of the string matched and C<< <y> >> the part not yet
t2
Done at position 4
-=head1 BUGS
-
-Code expressions, conditional expressions, and independent expressions
-are I<experimental>. Don't use them in production code. Yet.
-
=head1 SEE ALSO
This is just a tutorial. For the full story on Perl regular
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 2000 Mark Kvale
+Copyright (c) 2000 Mark Kvale.
All rights reserved.
+Now maintained by Perl porters.
This document may be distributed under the same terms as Perl itself.
https://perl5.git.perl.org / perl5.git / blobdiff