Regular Expressions::PCRE::What are the pattern modifiers
i (PCRE_CASELESS)
If this modifier 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 D modifier is set). This is the same as
Perl.
When this modifier 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 modifier. If there are no "\n" characters in a subject string,
or no occurrences of ^ or $ in a pattern, setting this modifier has no effect.
s (PCRE_DOTALL)
If this modifier is set, a dot metacharacter in the pattern matches all
characters, including newlines. Without it, newlines are excluded. This
modifier is equivalent to Perl's /s modifier. A negative class such as [^a]
always matches a newline character, independent of the setting of this
modifier.
x (PCRE_EXTENDED)
If this modifier 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 modifier, 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.
e
If this modifier is set, preg_replace() does normal substitution of backreferences in
the replacement string, evaluates it as PHP code, and uses the result for replacing
the search string. Single and double quotes are escaped by backslashes in
substituted backreferences.
Only preg_replace() uses this modifier; it is ignored by other PCRE functions.
Note: This modifier was not available in PHP 3.
A (PCRE_ANCHORED)
If this modifier 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.
D (PCRE_DOLLAR_ENDONLY)
If this modifier is set, a dollar metacharacter in the pattern matches only at
the end of the subject string. Without this modifier, a dollar also matches
immediately before the final character if it is a newline (but not before any
other newlines). This modifier is ignored if m modifier is set. There is no
equivalent to this modifier in Perl.
S
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
modifier 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 modifier 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) modifier setting within the pattern
or by a question mark behind a quantifier (e.g. .*?).
X (PCRE_EXTRA)
This modifier 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 modifier.
u (PCRE_UTF8)
This modifier turns on additional functionality of PCRE that is incompatible
with Perl. Pattern strings are treated as UTF-8. This modifier is available
from PHP 4.1.0 or greater on Unix and from PHP 4.2.3 on win32.