The current possible PCRE options are listed below. The names in parentheses refer to internal PCRE names for these options.
- i (PCRE_CASELESS)
If this option is set, letters in the pattern match both upper and lower case letters.
- m (PCRE_MULTILINE)
By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless E option is set). This is the same as Perl.
When this option is set, the "start of line" and "end of line" constructs match immediately following or immediately before any newline in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option. If there are no "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting this option has no effect.
- s (PCRE_DOTALL)
If this option is set, a dot metacharater in the pattern matches all characters, including newlines. Without it, newlines are excluded. This option is equivalent to Perl's /s option. A negative class such as [^a] always matches a newline character, independent of the setting of this option.
- x (PCRE_EXTENDED)
If this option is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x option, and makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.
If this option is set, preg_replace() does normal substitution of \\ references in the replacement string, evaluates it as PHP code, and uses the result for replacing the search string.
Only preg_replace() uses this option, it is ignored by other PCRE functions.
- A (PCRE_ANCHORED)
If this option is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the start of the string which is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.
- E (PCRE_DOLLAR_ENDONLY)
If this option is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before the final character if it is a newline (but not before any other newlines). This option is ignored if m option is set. There is no equivalent to this option in Perl.
When a pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. If this option is set, then this extra analysis is performed. At present, studying a pattern is useful only for non-anchored patterns that do not have a single fixed starting character.
- U (PCRE_UNGREEDY)
This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern.
- X (PCRE_EXTRA)
This option turns on additional functionality of PCRE that is incompatible with Perl. Any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. There are at present no other features controlled by this option.