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. Together with the capability of embedding comments described later, this goes a long way towards making Perl 5 a readable language. See the C comment deletion code in the perlop manpage .
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 $* , in case you have some (badly behaved) older code that sets it in another module.
The following standard quantifiers are recognized:
(If a curly bracket occurs in any other context, it is treated as a regular character.) The "*" modifier is equivalent to {0,} , the "+" modifier to {1,} , and the "?" modifier to {0,1} . There is no limit to the size of n or m, but large numbers will chew up more memory.
By default, a quantified subpattern is "greedy", that is, it will match as many times as possible without causing the rest pattern not to match. The standard quantifiers are all "greedy", in that they match as many occurrences as possible (given a particular starting location) without causing the pattern to fail. If you want it to match the minimum number of times possible, follow the quantifier with a "?" after any of them. Note that the meanings don't change, just the "gravity":
Since patterns are processed as double quoted strings, the following also work:
In addition, Perl defines the following:
Note that \w matches a single alphanumeric character, not a whole word. To match a word you'd need to say \w+ . You may use \w , \W , \s , \S , \d and \D within character classes (though not as either end of a range).
Perl defines the following zero-width assertions:
A word boundary ( \b ) is defined as a spot between two characters that has a \w on one side of it and and a \W on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a \W . (Within character classes \b represents backspace rather than a word boundary.) The \A and \Z are just like "^" and "$" except that they won't match multiple times when the /m modifier is used, while "^" and "$" will match at every internal line boundary.
When the bracketing construct ( ... ) is used, <digit> matches the digit'th substring. (Outside of the pattern, always use "$" instead of "\" in front of the digit. The scope of <digit> (and $` , $& , and $') extends to the end of the enclosing BLOCK or eval string, or to the next pattern match with subexpressions. 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 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 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:
is like
but doesn't spit out extra fields.
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, \x nn , where nn are hexidecimal digits, matches the character whose ASCII value is nn . The expression \c x 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 n th 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.