pcre Syntax

The pcre keyword accepts standard Perl-compatible regular expression (PCRE) syntax. The following sections describe that syntax.

Tip	While this section describes the basic syntax you may use for PCRE, you may want to consult an online reference or book dedicated to Perl and PCRE for more advanced information.

Metacharacters

Metacharacters are literal characters that have special meaning within regular expressions. When you use them within a regular expression, you must ��escape” them by preceding them with a backslash.

The following table describes the metacharacters you can use with PCRE and gives examples of each.

PCRE Metacharacters
Metacharacter	Description	Example
`.`	Matches any character except newlines. If `s` is used as a modifying option, it also includes newline characters.	`abc.` matches `abcd`, `abc1`, `abc#`, and so on.
`*`	Matches zero or more occurrences of a character or expression.	`abc*` matches `abc`, `abcc`, `abccc`, `abccccc`, and so on.
`?`	Matches zero or one occurrence of a character or expression.	`abc?` matches `abc`.
`+`	Matches one or more occurrences of a character or expression.	`abc+` matches `abc`, `abcc`, `abccc`, `abccccc`, and so on.
`()`	Groups expressions.	`(abc)+` matches `abc`, `abcabc`, `abcabcabc` and so on.
`{}`	Specifies a limit for the number of matches for a character or expression. If you want to set a lower and upper limit, separate the lower limit and upper limit with a comma.	`a{4,6}` matches `aaaa`, `aaaaa`, or `aaaaaa`. `(ab){2}` matches `abab`.
`[]`	Allows you to define character classes, and matches any character or combination of characters described in the set.	`[abc123]` matches `a` or `b` or `c`, and so on.
`^`	Matches content at the beginning of a string. Also used for negation, if used within a character class.	`^in` matches the “in” in `info`, but not in `bin`. `[^a]` matches anything that does not contain `a`.
`$`	Matches content at the end of a string.	`ce$` matches the “`ce`” in `announce`, but not `cent`.
`\|`	Indicates an OR expression.	`(MAILTO\|HELP)` matches `MAILTO` or `HELP`.
`\`	Allows you to use metacharacters as actual characters and is also used to specify a predefined character class.	`\.` matches a period, `\*` matches an asterisk, `\\` matches a backslash and so on. `\d` matches the numeric characters, `\w` matches alphanumeric characters, and so on.

Character Classes

Character classes include alphabetic characters, numeric characters, alphanumeric characters, and white space characters. While you can create your own character classes within brackets, you can use the predefined classes as shortcuts for different types of character types. When used without additional qualifiers, a character class matches a single digit or character.

The following table describes and provides examples of the predefined character classes accepted by PCRE.

PCRE Character Classes
Character Class	Description	Character Class Definition
\d	Matches a numeric character (“digit”).	[0-9]
\D	Matches anything that is not an numeric character.	[^0-9]
\w	Matches an alphanumeric character (“word”).	[a-zA-Z0-9_]
\W	Matches anything that is not an alphanumeric character.	[^a-zA-Z0-9_]
\s	Matches white space characters, including spaces, carriage returns, tabs, newlines, and form feeds.	[ \r\t\n\f]
\S	Matches anything that is not a white space character.	[^ \r\t\n\f]