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
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. If your string is better thought of as a sequence of arbitrary
+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. Here are some examples of escapes:
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'
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, these character classes match more
-than just a few characters in the ISO 8859-1 range.
+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.
=over 4
The period '.' matches any character but "\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.
+
=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.
+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
+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,
=over 4
-=item 0
+=item Z<>0
Start with the first letter in the string 'a'.
-=item 1
+=item Z<>1
Try the first alternative in the first group 'abd'.
-=item 2
+=item Z<>2
Match 'a' followed by 'b'. So far so good.
-=item 3
+=item Z<>3
'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 4
+=item Z<>4
Match 'a' followed by 'b' followed by 'c'. We are on a roll
and have satisfied the first group. Set $1 to 'abc'.
-=item 5
+=item Z<>5
Move on to the second group and pick the first alternative
'df'.
-=item 6
+=item Z<>6
Match the 'd'.
-=item 7
+=item Z<>7
'f' in the regexp doesn't match 'e' in the string, so a dead
end. Backtrack one character and pick the second alternative in the
second group 'd'.
-=item 8
+=item Z<>8
'd' matches. The second grouping is satisfied, so set $2 to
'd'.
-=item 9
+=item Z<>9
We are at the end of the regexp, so we are done! We have
matched 'abcd' out of the string "abcde".
=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
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 $exp: '${$exp}' at position ($-[$exp],$+[$exp])\n";
}
prints
In the second match, C<$`> 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
/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
=over 4
-=item 0
+=item Z<>0
Start with the first letter in the string 't'.
-=item 1
+=item Z<>1
The first quantifier '.*' starts out by matching the whole
string 'the cat in the hat'.
-=item 2
+=item Z<>2
'a' in the regexp element 'at' doesn't match the end of the
string. Backtrack one character.
-=item 3
+=item Z<>3
'a' in the regexp element 'at' still doesn't match the last
letter of the string 't', so backtrack one more character.
-=item 4
+=item Z<>4
Now we can match the 'a' and the 't'.
-=item 5
+=item Z<>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 6
+=item Z<>6
We are done!
extended C<//x> modifiers. There are a few more things you might
want to know about matching operators.
-=head3 Optimizing pattern evaluation
-
-We pointed out earlier that variables in regexps are substituted
-before the regexp is evaluated:
-
- $pattern = 'Seuss';
- while (<>) {
- print if /$pattern/;
- }
-
-This will print any lines containing the word C<Seuss>. It is not as
-efficient as it could be, however, because Perl has to re-evaluate
-(or compile) C<$pattern> each time through the loop. If C<$pattern> won't be
-changing over the lifetime of the script, we can add the C<//o>
-modifier, which directs Perl to only perform variable substitutions
-once:
-
- #!/usr/bin/perl
- # Improved simple_grep
- $regexp = shift;
- while (<>) {
- print if /$regexp/o; # a good deal faster
- }
-
-
=head3 Prohibiting substitution
If you change C<$pattern> after the first substitution happens, Perl
=head3 Global matching
-The final two modifiers C<//g> and C<//c> concern multiple matches.
+The final two modifiers we will discuss here,
+C<//g> and C<//c>, concern multiple matches.
The modifier C<//g> stands for global matching and allows the
matching operator to match within a string as many times as possible.
In scalar context, successive invocations against a string will have
we wanted just the words, we could use
@words = ($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
important not only to match what is desired, but to reject what is not
desired.
+(There are other regexp modifiers that are available, such as
+C<//o>, but their specialized uses are beyond the
+scope of this introduction. )
+
=head3 Search and replace
Regular expressions also play a big role in I<search and replace>
$regexp = shift;
$replacement = shift;
while (<>) {
- s/$regexp/$replacement/go;
+ s/$regexp/$replacement/g;
print;
}
^D
% simple_replace regexp regex perlretut.pod
In C<simple_replace> we used the C<s///g> modifier to replace all
-occurrences of the regexp on each line and the C<s///o> modifier to
-compile the regexp only once. As with C<simple_grep>, both the
-C<print> and the C<s/$regexp/$replacement/go> use C<$_> implicitly.
+occurrences of the regexp on each line. (Even though the regular
+expression appears in a loop, Perl is smart enough to compile it
+only once.) As with C<simple_grep>, both the
+C<print> and the C<s/$regexp/$replacement/g> use C<$_> implicitly.
If you don't want C<s///> to change your original variable you can use
the non-destructive substitute modifier, C<s///r>. This changes the
-behavior so that C<s///r> returns the final substituted string:
+behavior so that C<s///r> returns the final substituted string
+(instead of the number of substitutions):
$x = "I like dogs.";
$y = $x =~ s/dogs/cats/r;
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);
It does not protect C<$> or C<@>, so that variables can still be
substituted.
-C<\Q>, C<\L>, C<\U> and C<\E> are actually part of the syntax of regular
-expression I<literals>, and are not part of regexp syntax proper. So they
-do not work in interpolated patterns.
+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 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}>.
+\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}>.)
/\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 is found in the file NamesList.txt in the
-lib/perl5/X.X.X/unicore directory (where X.X.X is the perl
-version number as it is installed on your system).
-
-The answer to requirement 2), as of 5.6.0, is that a regexp uses Unicode
-characters. 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
+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 regex 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, most usually called
+"character properties". These are 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
+property, which is the negation of the C<\p{name}> one. 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
-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}>. Other sets are the Unicode blocks, the names
-of which begin with "In". One such block is dedicated to mathematical
-operators, and its pattern formula is <C\p{InMathematicalOperators>}>.
-For the full list see L<perluniprops>.
+(The "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, 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
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",
+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.
+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>
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). If
-Unicode is enabled (see C<perlunicode/The "Unicode Bug">),
-then these classes are defined the same as their
-corresponding Perl Unicode classes: C<[:upper:]> is the same as
-C<\p{IsUpper}>, etc. The C<[:digit:]>, C<[:word:]>, and
+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
+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
=head2 Compiling and saving regular expressions
-In Part 1 we discussed the C<//o> modifier, which compiles a regexp
-just once. This suggests that a compiled regexp is some data structure
+In Part 1 we mentioned that Perl compiles a regexp into a compact
+sequence of opcodes. Thus, a compiled regexp is a data structure
that can be stored once and used again and again. The regexp quote
C<qr//> does exactly that: C<qr/string/> compiles the C<string> as a
regexp and transforms the result into a form that can be assigned to a
$pattern = join '|', @regexp;
while ($line = <>) {
- print $line if $line =~ /$pattern/o;
+ print $line if $line =~ /$pattern/;
}
^D
% 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
This style of commenting has been largely superseded by the raw,
freeform commenting that is allowed with the C<//x> modifier.
-The modifiers 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,
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
$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
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<?{}>:
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${pat}bar/; # compiles ok
Another form of code expression is the I<pattern code expression>.
Note that the variables C<$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
combination with embedded code.
%count = ();
- "supercalifragilisticexpialidoceous" =~
- /([aeiou])(?{ $count{$1}++; })(*FAIL)/oi;
+ "supercalifragilisticexpialidocious" =~
+ /([aeiou])(?{ $count{$1}++; })(*FAIL)/i;
printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);
The pattern begins with a class matching a subset of letters. Whenever
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.)
use re '/m'; # or any other flags
$multiline_string =~ /^foo/; # /m is implied
-The C<re '/flags'> pragma turns on the given regular expression flags
-until the end of the lexical scope. See C<re/"'/flags' mode"> for more
+The C<re '/flags'> pragma (introduced in Perl
+5.14) turns on the given regular expression flags
+until the end of the lexical scope. See
+L<re/"'E<sol>flags' mode"> for more
detail.
use re 'debug';
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)
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
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