A regular expression is a pattern that is matched against a
subject string from left to right. Most characters stand for
themselves in a pattern, and match the corresponding characters
in the subject. As a trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to
itself. When caseless matching is specified (the PCRE_CASELESS
option), letters are matched independently of case. In a UTF
mode, PCRE always understands the concept of case for characters
whose values are less than 128, so caseless matching is always
possible. For characters with higher values, the concept of case
is supported if PCRE is compiled with Unicode property support,
but not otherwise. If you want to use caseless matching for
characters 128 and above, you must ensure that PCRE is compiled
with Unicode property support as well as with UTF support.
The power of regular expressions comes from the ability to
include alternatives and repetitions in the pattern. These are
encoded in the pattern by the use of metacharacters, which do not
stand for themselves but instead are interpreted in some special
way.
There are two different sets of metacharacters: those that are
recognized anywhere in the pattern except within square brackets,
and those that are recognized within square brackets. Outside
square brackets, the metacharacters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a
"character class". In a character class the only metacharacters
are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the
metacharacters.
