=head1 DESCRIPTION
-For a description of how to use regular expressions in matching
-operations, see C<m//> and C<s///> in L<perlop>. The matching operations can
-have various modifiers, some of which relate to the interpretation of
-the regular expression inside. These are:
+This page describes the syntax of regular expressions in Perl. For a
+description of how to I<use> regular expressions in matching
+operations, plus various examples of the same, see C<m//> and C<s///> in
+L<perlop>.
- i Do case-insensitive pattern matching.
- m Treat string as multiple lines.
- s Treat string as single line.
- x Use extended regular expressions.
+The matching operations can have various modifiers. The modifiers
+which relate to the interpretation of the regular expression inside
+are listed below. For the modifiers that alter the behaviour of the
+operation, see L<perlop/"m//"> and L<perlop/"s//">.
+
+=over 4
+
+=item i
+
+Do case-insensitive pattern matching.
+
+If C<use locale> is in effect, the case map is taken from the current
+locale. See L<perllocale>.
+
+=item m
+
+Treat string as multiple lines. That is, change "^" and "$" from matching
+at only the very start or end of the string to the start or end of any
+line anywhere within the string,
+
+=item s
+
+Treat string as single line. That is, change "." to match any character
+whatsoever, even a newline, which it normally would not match.
+
+=item x
+
+Extend your pattern's legibility by permitting whitespace and comments.
+
+=back
These are usually written as "the C</x> modifier", even though the delimiter
in question might not actually be a slash. In fact, any of these
the new C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
-the regular expression parser to ignore whitespace that is not
-backslashed or within a character class. You can use this to break up
+the regular expression parser to ignore whitespace that is neither
+backslashed nor within a character class. You can use this to break up
your regular expression into (slightly) more readable parts. The C<#>
character is also treated as a metacharacter introducing a comment,
-just as in ordinary Perl code. Taken together, these features go a
-long way towards making Perl 5 a readable language. See the C comment
-deletion code in L<perlop>.
+just as in ordinary Perl code. This also means that if you want real
+whitespace or C<#> characters in the pattern that you'll have to either
+escape them or encode them using octal or hex escapes. Taken together,
+these features go a long way towards making Perl's regular expressions
+more readable. See the C comment deletion code in L<perlop>.
=head2 Regular Expressions
\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
- $ Match the end of the line
+ $ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match only at the
-beginning of the string, the "$" character only at the end (or before the
+By default, the "^" character is guaranteed to match at only the
+beginning of the string, the "$" character at only the end (or before the
newline at the end) and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines
will not be matched by "^" or "$". You may, however, wish to treat a
newline within the string, and "$" will match before any newline. At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator. (Older programs did this by setting C<$*>,
-but this practice is deprecated in Perl 5.)
+but this practice is now deprecated.)
To facilitate multi-line substitutions, the "." character never matches a
-newline unless you use the C</s> modifier, which tells Perl to pretend
+newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't. The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
code that sets it in another module.
(If a curly bracket occurs in any other context, it is treated
as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
-modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. There is no limit to the
-size of n or m, but large numbers will chew up more memory.
+modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
+to integral values less than 65536.
By default, a quantified subpattern is "greedy", that is, it will match as
-many times as possible without causing the rest pattern not to match. The
-standard quantifiers are all "greedy", in that they match as many
-occurrences as possible (given a particular starting location) without
-causing the pattern to fail. If you want it to match the minimum number
-of times possible, follow the quantifier with a "?" after any of them.
-Note that the meanings don't change, just the "gravity":
+many times as possible (given a particular starting location) while still
+allowing the rest of the pattern to match. If you want it to match the
+minimum number of times possible, follow the quantifier with a "?". Note
+that the meanings don't change, just the "greediness":
*? Match 0 or more times
+? Match 1 or more times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
-Since patterns are processed as double quoted strings, the following
+Because patterns are processed as double quoted strings, the following
also work:
- \t tab
- \n newline
- \r return
- \f form feed
- \v vertical tab, whatever that is
- \a alarm (bell)
- \e escape
- \033 octal char
- \x1b hex char
+ \t tab (HT, TAB)
+ \n newline (LF, NL)
+ \r return (CR)
+ \f form feed (FF)
+ \a alarm (bell) (BEL)
+ \e escape (think troff) (ESC)
+ \033 octal char (think of a PDP-11)
+ \x1B hex char
\c[ control char
- \l lowercase next char
- \u uppercase next char
- \L lowercase till \E
- \U uppercase till \E
- \E end case modification
+ \l lowercase next char (think vi)
+ \u uppercase next char (think vi)
+ \L lowercase till \E (think vi)
+ \U uppercase till \E (think vi)
+ \E end case modification (think vi)
\Q quote regexp metacharacters till \E
+If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
+and <\U> is taken from the current locale. See L<perllocale>.
+
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
\D Match a non-digit character
Note that C<\w> matches a single alphanumeric character, not a whole
-word. To match a word you'd need to say C<\w+>. You may use C<\w>, C<\W>, C<\s>,
-C<\S>, C<\d> and C<\D> within character classes (though not as either end of a
-range).
+word. To match a word you'd need to say C<\w+>. If C<use locale> is in
+effect, the list of alphabetic characters generated by C<\w> is taken
+from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
+C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
+either end of a range).
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match a non-(word boundary)
- \A Match only at beginning of string
- \Z Match only at end of string
- \G Match only where previous m//g left off
+ \A Match at only beginning of string
+ \Z Match at only end of string (or before newline at the end)
+ \G Match only where previous m//g left off (works only with /g)
A word boundary (C<\b>) is defined as a spot between two characters that
-has a C<\w> on one side of it and and a C<\W> on the other side of it (in
+has a C<\w> on one side of it and a C<\W> on the other side of it (in
either order), counting the imaginary characters off the beginning and
end of the string as matching a C<\W>. (Within character classes C<\b>
represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
just like "^" and "$" except that they won't match multiple times when the
C</m> modifier is used, while "^" and "$" will match at every internal line
-boundary.
-
-When the bracketing construct C<( ... )> is used, \<digit> matches the
-digit'th substring. (Outside of the pattern, always use "$" instead of
-"\" in front of the digit. The scope of $<digit> (and C<$`>, C<$&>, and C<$')>
-extends to the end of the enclosing BLOCK or eval string, or to the
-next successful pattern match, whichever comes first.
-If you want to
-use parentheses to delimit subpattern (e.g. a set of alternatives) without
-saving it as a subpattern, follow the ( with a ?.
-The \<digit> notation
-sometimes works outside the current pattern, but should not be relied
-upon.) You may have as many parentheses as you wish. If you have more
+boundary. To match the actual end of the string, not ignoring newline,
+you can use C<\Z(?!\n)>. The C<\G> assertion can be used to chain global
+matches (using C<m//g>), as described in
+L<perlop/"Regexp Quote-Like Operators">.
+
+It is also useful when writing C<lex>-like scanners, when you have several
+regexps which you want to match against consequent substrings of your
+string, see the previous reference.
+The actual location where C<\G> will match can also be influenced
+by using C<pos()> as an lvalue. See L<perlfunc/pos>.
+
+When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
+digit'th substring. Outside of the pattern, always use "$" instead of "\"
+in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
+outside the current pattern, this should not be relied upon. See the
+WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
+extends to the end of the enclosing BLOCK or eval string, or to the next
+successful pattern match, whichever comes first. If you want to use
+parentheses to delimit a subpattern (e.g., a set of alternatives) without
+saving it as a subpattern, follow the ( with a ?:.
+
+You may have as many parentheses as you wish. If you have more
than 9 substrings, the variables $10, $11, ... refer to the
corresponding substring. Within the pattern, \10, \11, etc. refer back
-to substrings if there have been at least that many left parens before
-the backreference. Otherwise (for backward compatibilty) \10 is the
+to substrings if there have been at least that many left parentheses before
+the backreference. Otherwise (for backward compatibility) \10 is the
same as \010, a backspace, and \11 the same as \011, a tab. And so
on. (\1 through \9 are always backreferences.)
C<$+> returns whatever the last bracket match matched. C<$&> returns the
-entire matched string. ($0 used to return the same thing, but not any
+entire matched string. (C<$0> used to return the same thing, but not any
more.) C<$`> returns everything before the matched string. C<$'> returns
everything after the matched string. Examples:
$seconds = $3;
}
+Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
+the program, it has to provide them on each and every pattern match.
+This can slow your program down. The same mechanism that handles
+these provides for the use of $1, $2, etc., so you pay the same price
+for each regexp that contains capturing parentheses. But if you never
+use $&, etc., in your script, then regexps I<without> capturing
+parentheses won't be penalized. So avoid $&, $', and $` if you can,
+but if you can't (and some algorithms really appreciate them), once
+you've used them once, use them at will, because you've already paid
+the price.
+
You will note that all backslashed metacharacters in Perl are
alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression
languages, there are no backslashed symbols that aren't alphanumeric.
-So anything that looks like \\, \(, \), \<, \>, \{, or \} is always
+So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always
interpreted as a literal character, not a metacharacter. This makes it
simple to quote a string that you want to use for a pattern but that
-you are afraid might contain metacharacters. Simply quote all the
+you are afraid might contain metacharacters. Quote simply all the
non-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
-You can also use the built-in quotemeta() function to do this.
+You can also use the builtin quotemeta() function to do this.
An even easier way to quote metacharacters right in the match operator
-is to say
+is to say
/$unquoted\Q$quoted\E$unquoted/
-Perl 5 defines a consistent extension syntax for regular expressions.
-The syntax is a pair of parens with a question mark as the first thing
-within the parens (this was a syntax error in Perl 4). The character
-after the question mark gives the function of the extension. Several
-extensions are already supported:
+Perl defines a consistent extension syntax for regular expressions.
+The syntax is a pair of parentheses with a question mark as the first
+thing within the parentheses (this was a syntax error in older
+versions of Perl). The character after the question mark gives the
+function of the extension. Several extensions are already supported:
=over 10
=item (?#text)
-A comment. The text is ignored.
+A comment. The text is ignored. If the C</x> switch is used to enable
+whitespace formatting, a simple C<#> will suffice.
=item (?:regexp)
-This groups things like "()" but doesn't make backrefences like "()" does. So
+This groups things like "()" but doesn't make backreferences like "()" does. So
split(/\b(?:a|b|c)\b/)
"bar" that is preceded by something which is not "foo". That's because
the C<(?!foo)> is just saying that the next thing cannot be "foo"--and
it's not, it's a "bar", so "foobar" will match. You would have to do
-something like C</(?foo)...bar/> for that. We say "like" because there's
+something like C</(?!foo)...bar/> for that. We say "like" because there's
the case of your "bar" not having three characters before it. You could
-cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
+cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
easier just to say:
- if (/foo/ && $` =~ /bar$/)
+ if (/foo/ && $` =~ /bar$/)
=item (?imsx)
One or more embedded pattern-match modifiers. This is particularly
useful for patterns that are specified in a table somewhere, some of
which want to be case sensitive, and some of which don't. The case
-insensitive ones merely need to include C<(?i)> at the front of the
+insensitive ones need to include merely C<(?i)> at the front of the
pattern. For example:
$pattern = "foobar";
- if ( /$pattern/i )
+ if ( /$pattern/i )
# more flexible:
$pattern = "(?i)foobar";
- if ( /$pattern/ )
+ if ( /$pattern/ )
=back
regular expressions, and 2) whenever you see one, you should stop
and "question" exactly what is going on. That's psychology...
+=head2 Backtracking
+
+A fundamental feature of regular expression matching involves the notion
+called I<backtracking>. which is used (when needed) by all regular
+expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and
+C<{n,m}?>.
+
+For a regular expression to match, the I<entire> regular expression must
+match, not just part of it. So if the beginning of a pattern containing a
+quantifier succeeds in a way that causes later parts in the pattern to
+fail, the matching engine backs up and recalculates the beginning
+part--that's why it's called backtracking.
+
+Here is an example of backtracking: Let's say you want to find the
+word following "foo" in the string "Food is on the foo table.":
+
+ $_ = "Food is on the foo table.";
+ if ( /\b(foo)\s+(\w+)/i ) {
+ print "$2 follows $1.\n";
+ }
+
+When the match runs, the first part of the regular expression (C<\b(foo)>)
+finds a possible match right at the beginning of the string, and loads up
+$1 with "Foo". However, as soon as the matching engine sees that there's
+no whitespace following the "Foo" that it had saved in $1, it realizes its
+mistake and starts over again one character after where it had the
+tentative match. This time it goes all the way until the next occurrence
+of "foo". The complete regular expression matches this time, and you get
+the expected output of "table follows foo."
+
+Sometimes minimal matching can help a lot. Imagine you'd like to match
+everything between "foo" and "bar". Initially, you write something
+like this:
+
+ $_ = "The food is under the bar in the barn.";
+ if ( /foo(.*)bar/ ) {
+ print "got <$1>\n";
+ }
+
+Which perhaps unexpectedly yields:
+
+ got <d is under the bar in the >
+
+That's because C<.*> was greedy, so you get everything between the
+I<first> "foo" and the I<last> "bar". In this case, it's more effective
+to use minimal matching to make sure you get the text between a "foo"
+and the first "bar" thereafter.
+
+ if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
+ got <d is under the >
+
+Here's another example: let's say you'd like to match a number at the end
+of a string, and you also want to keep the preceding part the match.
+So you write this:
+
+ $_ = "I have 2 numbers: 53147";
+ if ( /(.*)(\d*)/ ) { # Wrong!
+ print "Beginning is <$1>, number is <$2>.\n";
+ }
+
+That won't work at all, because C<.*> was greedy and gobbled up the
+whole string. As C<\d*> can match on an empty string the complete
+regular expression matched successfully.
+
+ Beginning is <I have 2 numbers: 53147>, number is <>.
+
+Here are some variants, most of which don't work:
+
+ $_ = "I have 2 numbers: 53147";
+ @pats = qw{
+ (.*)(\d*)
+ (.*)(\d+)
+ (.*?)(\d*)
+ (.*?)(\d+)
+ (.*)(\d+)$
+ (.*?)(\d+)$
+ (.*)\b(\d+)$
+ (.*\D)(\d+)$
+ };
+
+ for $pat (@pats) {
+ printf "%-12s ", $pat;
+ if ( /$pat/ ) {
+ print "<$1> <$2>\n";
+ } else {
+ print "FAIL\n";
+ }
+ }
+
+That will print out:
+
+ (.*)(\d*) <I have 2 numbers: 53147> <>
+ (.*)(\d+) <I have 2 numbers: 5314> <7>
+ (.*?)(\d*) <> <>
+ (.*?)(\d+) <I have > <2>
+ (.*)(\d+)$ <I have 2 numbers: 5314> <7>
+ (.*?)(\d+)$ <I have 2 numbers: > <53147>
+ (.*)\b(\d+)$ <I have 2 numbers: > <53147>
+ (.*\D)(\d+)$ <I have 2 numbers: > <53147>
+
+As you see, this can be a bit tricky. It's important to realize that a
+regular expression is merely a set of assertions that gives a definition
+of success. There may be 0, 1, or several different ways that the
+definition might succeed against a particular string. And if there are
+multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve.
+
+When using lookahead assertions and negations, this can all get even
+tricker. Imagine you'd like to find a sequence of non-digits not
+followed by "123". You might try to write that as
+
+ $_ = "ABC123";
+ if ( /^\D*(?!123)/ ) { # Wrong!
+ print "Yup, no 123 in $_\n";
+ }
+
+But that isn't going to match; at least, not the way you're hoping. It
+claims that there is no 123 in the string. Here's a clearer picture of
+why it that pattern matches, contrary to popular expectations:
+
+ $x = 'ABC123' ;
+ $y = 'ABC445' ;
+
+ print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
+ print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
+
+ print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
+ print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
+
+This prints
+
+ 2: got ABC
+ 3: got AB
+ 4: got ABC
+
+You might have expected test 3 to fail because it seems to a more
+general purpose version of test 1. The important difference between
+them is that test 3 contains a quantifier (C<\D*>) and so can use
+backtracking, whereas test 1 will not. What's happening is
+that you've asked "Is it true that at the start of $x, following 0 or more
+non-digits, you have something that's not 123?" If the pattern matcher had
+let C<\D*> expand to "ABC", this would have caused the whole pattern to
+fail.
+The search engine will initially match C<\D*> with "ABC". Then it will
+try to match C<(?!123> with "123" which, of course, fails. But because
+a quantifier (C<\D*>) has been used in the regular expression, the
+search engine can backtrack and retry the match differently
+in the hope of matching the complete regular expression.
+
+Well now,
+the pattern really, I<really> wants to succeed, so it uses the
+standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
+time. Now there's indeed something following "AB" that is not
+"123". It's in fact "C123", which suffices.
+
+We can deal with this by using both an assertion and a negation. We'll
+say that the first part in $1 must be followed by a digit, and in fact, it
+must also be followed by something that's not "123". Remember that the
+lookaheads are zero-width expressions--they only look, but don't consume
+any of the string in their match. So rewriting this way produces what
+you'd expect; that is, case 5 will fail, but case 6 succeeds:
+
+ print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
+ print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
+
+ 6: got ABC
+
+In other words, the two zero-width assertions next to each other work like
+they're ANDed together, just as you'd use any builtin assertions: C</^$/>
+matches only if you're at the beginning of the line AND the end of the
+line simultaneously. The deeper underlying truth is that juxtaposition in
+regular expressions always means AND, except when you write an explicit OR
+using the vertical bar. C</ab/> means match "a" AND (then) match "b",
+although the attempted matches are made at different positions because "a"
+is not a zero-width assertion, but a one-width assertion.
+
+One warning: particularly complicated regular expressions can take
+exponential time to solve due to the immense number of possible ways they
+can use backtracking to try match. For example this will take a very long
+time to run
+
+ /((a{0,5}){0,5}){0,5}/
+
+And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
+it would take literally forever--or until you ran out of stack space.
+
=head2 Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8 regexp
Any single character matches itself, unless it is a I<metacharacter>
with a special meaning described here or above. You can cause
characters which normally function as metacharacters to be interpreted
-literally by prefixing them with a "\" (e.g. "\." matches a ".", not any
+literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
character; "\\" matches a "\"). A series of characters matches that
series of characters in the target string, so the pattern C<blurfl>
would match "blurfl" in the target string.
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
of octal digits, matches the character whose ASCII value is I<nnn>.
-Similarly, \xI<nn>, where I<nn> are hexidecimal digits, matches the
+Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
character whose ASCII value is I<nn>. The expression \cI<x> matches the
ASCII character control-I<x>. Finally, the "." metacharacter matches any
character except "\n" (unless you use C</s>).
Within a pattern, you may designate subpatterns for later reference by
enclosing them in parentheses, and you may refer back to the I<n>th
-subpattern later in the pattern using the metacharacter \I<n>.
+subpattern later in the pattern using the metacharacter \I<n>.
Subpatterns are numbered based on the left to right order of their
opening parenthesis. Note that a backreference matches whatever
actually matched the subpattern in the string being examined, not the
rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
-match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1
+match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1
actually matched "0x", even though the rule C<0|0x> could
potentially match the leading 0 in the second number.
+
+=head2 WARNING on \1 vs $1
+
+Some people get too used to writing things like
+
+ $pattern =~ s/(\W)/\\\1/g;
+
+This is grandfathered for the RHS of a substitute to avoid shocking the
+B<sed> addicts, but it's a dirty habit to get into. That's because in
+PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
+the usual double-quoted string means a control-A. The customary Unix
+meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
+of doing that, you get yourself into trouble if you then add an C</e>
+modifier.
+
+ s/(\d+)/ \1 + 1 /eg;
+
+Or if you try to do
+
+ s/(\d+)/\1000/;
+
+You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
+C<${1}000>. Basically, the operation of interpolation should not be confused
+with the operation of matching a backreference. Certainly they mean two
+different things on the I<left> side of the C<s///>.
+
+=head2 SEE ALSO
+
+"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.
projects / p5sagit/p5-mst-13.2.git / blobdiff