The matching operations can have various modifiers, some of which relate to the interpretation of the regular expression inside. These are:
If use locale is in effect, the case map is taken from the current locale. See the perllocale manpage.
The /x modifier itself needs a little more explanation. It tells 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 #
character is also treated as a meta-character introducing a comment, just
as in ordinary Perl code. This also means that if you want real whitespace
or # 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
the perlop manpage.
In particular the following metacharacters have their standard egrep-ish meanings:
\ Quote the next meta-character
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Character class
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 string as a
multi-line buffer, such that the ``^'' will match after any 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 $*, but this practice is now deprecated.)
To facilitate multi-line substitutions, the ``.'' character never matches a
newline unless you use the /s modifier, which in effect tells Perl to pretend the string is a single
line--even if it isn't. The /s modifier also overrides the setting of $*, in case you have some (badly behaved) older code that sets it in another
module.
The following standard quantifiers are recognized:
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
(If a curly bracket occurs in any other context, it is treated as a regular
character.) The ``*'' modifier is equivalent to {0,}, the ``+'' modifier to {1,}, and the ``?'' modifier to {0,1}. n and m are limited to integral values less than 65536.
By default, a quantified sub-pattern is ``greedy'', that is, it will match as many times as possible without causing the rest of the 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'':
*? Match 0 or more times
+? Match 1 or more times
?? Match 0 or 1 time
{n}? Match exactly n times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
Because patterns are processed as double quoted strings, the following also work:
\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 (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 use locale is in effect, the case map used by \l, \L, \u
and <
\U> is taken from the current locale. See the perllocale manpage.
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
\W Match a non-word character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
\D Match a non-digit character
Note that \w matches a single alphanumeric character, not a whole word. To match a word
you'd need to say \w+. If use locale is in effect, the list of alphabetic characters generated by \w is taken from the current locale. See the perllocale manpage. You may use \w, \W,
\s, \S, \d, and \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 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
A word boundary (\b) is defined as a spot between two characters that has a \w on one side of it and and a \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 \W. (Within character classes \b
represents backspace rather than a word boundary.) The \A and \Z are just like ``^'' and ``$'' except that they won't match multiple times
when the
/m modifier is used, while ``^'' and ``$'' will match at every internal line
boundary. To match the actual end of the string, not ignoring newline, you
can use \Z. The \G assertion can be used to mix global matches (using m//g) and non-global ones, as described in
Regexp Quote-Like Operators. It is also useful when writing lex-like scanners, when you have several regexps which you want to match
against consequent substrings of your string, see the previous reference.
The actual location where \G will match can also be influenced by using pos as an lvalue. See pos.
When the bracketing construct is used, \<digit> matches the digit'th substring. Outside of the pattern, always use ``$''
instead of ``\'' in front of the digit. (While the \<digit> notation can on rare occasion work outside the current pattern, this should not be relied upon. See the
WARNING below.) The scope of $
<digit> (and $`, $&, and $') 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 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.)
$+ returns whatever the last bracket match matched. $& returns the entire matched string. ($0 used to return the same thing, but not any more.) $` returns everything before the matched string. $' returns everything after the matched string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
You will note that all backslashed metacharacters in Perl are alphanumeric,
such as \b, \w, \n. Unlike some other regular expression languages, there are no backslashed
symbols that aren't alphanumeric. So anything that looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a literal character, not a
meta-character. This makes it simple to quote a string that you want to use
for a pattern but that 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.
An even easier way to quote metacharacters right in the match operator is
to say
/$unquoted\Q$quoted\E$unquoted/
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:
# will suffice.
split(/\b(?:a|b|c)\b/)
is like
split(/\b(a|b|c)\b/)
but doesn't spit out extra fields.
/\w+/
matches a word followed by a tab, without including the tab in $&.
/foo/
matches any occurrence of ``foo'' that isn't followed by ``bar''. Note however that lookahead and lookbehind are
NOT the same thing. You cannot use this for lookbehind:
/bar/ will not find an occurrence of ``bar'' that is preceded by something which
is not ``foo''. That's because the /...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: /(?:...|^..?)bar/. Sometimes it's still easier just to say:
if (/foo/ && $` =~ /bar$/)
$pattern = "foobar";
if ( /$pattern/i )
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ )
*, *?, +, +?, {n,m}, and
{n,m}?.
For a regular expression to match, the 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 (\b) 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 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 .* was greedy, so you get everything between the
first ``foo'' and the 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 .* was greedy and gobbled up the whole string. As \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 (\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 \D* expand to
``ABC'', this would have caused the whole pattern to
fail. The search engine will initially match \D* with
``ABC''. Then it will try to match (?!123 with ``123'' which, of course, fails. But because a quantifier (\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, really wants to succeed, so it uses the standard regexp back-off-and-retry and
lets \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 $y =~ /^(\D*)(?=\d)(?!123)/ ;
6: got ABC
In other words, the two zero-width assertions next to each other work like
they're ANDed together, just as you'd use any builtin assertions: /^$/
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.
/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 *'s instead of limiting it to 0 through 5 matches, then it would take
literally forever--or until you ran out of stack space.
Any single character matches itself, unless it is a meta-character
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 character; ``\\'' matches a ``\'').
A series of characters matches that series of characters in the target string, so the pattern
blurfl
would match ``blurfl'' in the target string.
You can specify a character class, by enclosing a list of characters in [], which will match any one of the characters in the list. If the first
character after the ``['' is ``^'', the class matches any character not in
the list. Within a list, the ``-'' character is used to specify a range, so
that a-z represents all the characters between ``a'' and ``z'', inclusive.
Characters may be specified using a meta-character syntax much like that used in C: ``\n'' matches a newline, ``\t'' a tab, ``\r'' a carriage return, ``\f'' a form feed, etc. More generally, \ nnn, where nnn is a string of octal digits, matches the character whose ASCII value is nnn. Similarly, \xnn, where nn are hexadecimal digits, matches the character whose ASCII value is nn. The expression \cx matches the ASCII character control-x. Finally, the ``.'' meta-character matches any character except ``\n'' (unless you use /s).
You can specify a series of alternatives for a pattern using ``|'' to
separate them, so that fee|fie|foe will match any of ``fee'', ``fie'', or ``foe'' in the target string (as
would fe). Note that the first alternative includes everything from the last
pattern delimiter (``('', ``['', or the beginning of the pattern) up to the
first ``|'', and the last alternative contains everything from the last
``|'' to the next pattern delimiter. For this reason, it's common practice
to include alternatives in parentheses, to minimize confusion about where
they start and end. Note however that ``|'' is interpreted as a literal
with square brackets, so if you write [fee|fie|foe] you're really only matching [feio|].
Within a pattern, you may designate sub-patterns for later reference by
enclosing them in parentheses, and you may refer back to the nth sub-pattern later in the pattern using the meta-character \n. Sub-patterns are numbered based on the left to right order of their
opening parenthesis. Note that a backreference matches whatever actually
matched the sub-pattern in the string being examined, not the rules for
that sub-pattern. Therefore, \d*\s\1\d* will match ``0x1234 0x4321'',but not ``0x1234 01234'', because sub-pattern
1 actually matched ``0x'', even though the rule 0|0x could potentially match the leading 0 in the second number.
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the
RHS of a substitute to avoid shocking the
sed addicts, but it's a dirty habit to get into. That's because in PerlThink,
the righthand side of a s/// is a double-quoted string. \1 in the usual double-quoted string means a control-A. The customary Unix
meaning of \1 is kludged in for s///. However, if you get into the habit of doing that, you get yourself into
trouble if you then add an /e
modifier.
s/(\d+)/ \1 + 1 /eg;
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying \{1}000, whereas you can fix it with
${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 left side of the s///.