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 C<m> C<u> C<s> C<h> C<r> C<o> C<o> C<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.
+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
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
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'
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".
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
$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!
|\.\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,
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 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
+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,
$x = "BOB";
(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}>.
+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.
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>
% 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
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<?{}>:
Guessed: match at offset 0
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'
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'
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