NAME

DESCRIPTION

The matching operations can have various modifiers, some of which relate to the interpretation of the regular expression inside. These are:

These are usually written as ``the /x modifier'', even though the delimiter in question might not actually be a slash. In fact, any of these modifiers may also be embedded within the regular expression itself using the new (?...) construct. See below.

The /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 your regular expression into (slightly) more readable parts. The # 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 the perlop manpage .

Regular Expressions

In particular the following metacharacters have their standard egrep-ish meanings:

By default, the ``^'' character is guaranteed to match only at the beginning of the string, the ``$'' character only at the end (or before the newline at the end) and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by ``^'' or ``$''. 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 deprecated in Perl 5.)

To facilitate multi-line substitutions, the ``.'' character never matches a newline unless you use the /s modifier, which 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:

(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 subpattern 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'':

Since patterns are processed as double quoted strings, the following also work:

In addition, Perl defines the following:

Note that \w matches a single alphanumeric character, not a whole word. To match a word you'd need to say \w+. 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:

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(?!\n).

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 parens 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:

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 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 non-alphanumeric characters:

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

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:

(?#text)

A comment. The text is ignored. If the /x switch is used to enable whitespace formatting, a simple # will suffice.

(?:regexp)

This groups things like ``()'' but doesn't make backreferences like ``()'' does. So

split(/\b(?:a|b|c)\b/)

is like

split(/\b(a|b|c)\b/)

but doesn't spit out extra fields.

(?=regexp)

A zero-width positive lookahead assertion. For example, /\w+(?=\t)/ matches a word followed by a tab, without including the tab in $&.

(?!regexp)

A zero-width negative lookahead assertion. For example /foo(?!bar)/ 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: /(?!foo)bar/ will not find an occurrence of ``bar'' that is preceded by something which is not ``foo''. That's because the (?!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 /(?!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: /(?:(?!foo)...|^..?)bar/. Sometimes it's still easier just to say:

if (/foo/ && $` =~ /bar$/)

(?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 (?i) at the front of the pattern. For example:

$pattern = "foobar"; if ( /$pattern/i ) # more flexible: $pattern = "(?i)foobar"; if ( /$pattern/ )

Backtracking

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.'':

When the match runs, the first part of the regular expression (\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 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:

Which perhaps unexpectedly yields:

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.

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:

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.

Here are some variants, most of which don't work:

That will print out:

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 in order 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 nondigits not followed by ``123''. You might try to write that as

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:

This prints

You might have expected test 3 to fail because it just 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 nondigits, 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 backoff-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:

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

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.

Version 8 Regular Expressions

Any single character matches itself, unless it is a 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 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 metacharacter 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 ``.'' metacharacter 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 f(e|i|o)e). 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 subpatterns for later reference by enclosing them in parentheses, and you may refer back to the nth subpattern later in the pattern using the metacharacter \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, (0|0x)\d*\s\1\d* will match ``0x1234 0x4321'',but not ``0x1234 01234'', since subpattern 1 actually matched ``0x'', even though the rule 0|0x could potentially match the leading 0 in the second number.

WARNING on \1 vs $1

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 right-hand 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.

Or if you try to do

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///.