Escape sequences

The backslash character has several uses. Firstly,
if it is followed by a non-alphanumeric character, it takes away
any special meaning that character may have. This use of backslash
as an escape character applies both inside and outside character
classes.

For example, if you want to match a “*” character,
you write “\*” in the pattern. This applies whether or not the
following character would otherwise be interpreted as a
meta-character, so it is always safe to precede a non-alphanumeric
with “\” to specify that it stands for itself. In particular, if
you want to match a backslash, you write “\\”.

Note:

Single and double quoted PHP strings have special meaning of backslash. Thus if \ has
to be matched with a regular expression \\, then “\\\\” or ‘\\\\’
must be used in PHP code.

If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the pattern (other
than in a character class) and characters between a “#” outside a
character class and the next newline character are ignored. An
escaping backslash can be used to include a whitespace or “#”
character as part of the pattern.

A second use of backslash provides a way of
encoding non-printing characters in patterns in a visible manner.
There is no restriction on the appearance of non-printing
characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually
easier to use one of the following escape sequences than the binary
character it represents:

\a

alarm, that is, the BEL character (hex
07)

\cx

“control-x”, where x is any
character

\e

escape (hex 1B)

\f

formfeed (hex 0C)

\n

newline (hex 0A)

\p{xx}

a character with the xx property, see
unicode
properties for more info

\P{xx}

a character without the xx property, see
unicode
properties for more info

\r

carriage return (hex 0D)

\R

line break: matches \n, \r and
\r\n

\t

tab (hex 09)

\xhh

character with hex code hh

\ddd

character with octal code ddd, or
backreference

The precise effect of “\cx” is as follows:
if “x” is a lower case letter, it is converted to upper
case. Then bit 6 of the character (hex 40) is inverted. Thus
“\cz” becomes hex 1A, but “\c{” becomes hex 3B,
while “\c;” becomes hex 7B.

After “\x“, up to two hexadecimal digits
are read (letters can be in upper or lower case). In UTF-8 mode, “\x{…}” is allowed, where
the contents of the braces is a string of hexadecimal digits. It is
interpreted as a UTF-8 character whose code number is the given
hexadecimal number. The original hexadecimal escape sequence,
\xhh, matches a two-byte UTF-8 character if the value is
greater than 127.

After “\0” up to two further octal digits
are read. In both cases, if there are fewer than two digits, just
those that are present are used. Thus the sequence
“\0\x\07” specifies two binary zeros followed by a BEL
character. Make sure you supply two digits after the initial zero
if the character that follows is itself an octal digit.

The handling of a backslash followed by a digit
other than 0 is complicated. Outside a character class, PCRE reads
it and any following digits as a decimal number. If the number is
less than 10, or if there have been at least that many previous
capturing left parentheses in the expression, the entire sequence
is taken as a back reference. A
description of how this works is given later, following the
discussion of parenthesized subpatterns.

Inside a character class, or if the decimal number
is greater than 9 and there have not been that many capturing
subpatterns, PCRE re-reads up to three octal digits following the
backslash, and generates a single byte from the least significant 8
bits of the value. Any subsequent digits stand for themselves. For
example:

\040

is another way of writing a
space

\40

is the same, provided there are fewer
than 40 previous capturing subpatterns

\7

is always a back reference

\11

might be a back reference, or another way
of writing a tab

\011

is always a tab

\0113

is a tab followed by the character
“3”

\113

is the character with octal code 113
(since there can be no more than 99 back references)

\377

is a byte consisting entirely of 1
bits

\81

is either a back reference, or a binary
zero followed by the two characters “8” and “1”

Note that octal values of 100 or greater must not
be introduced by a leading zero, because no more than three octal
digits are ever read.

All the sequences that define a single byte value
can be used both inside and outside character classes. In addition,
inside a character class, the sequence “\b” is interpreted
as the backspace character (hex 08). Outside a character class it
has a different meaning (see below).

The third use of backslash is for specifying
generic character types:

\d

any decimal digit

\D

any character that is not a decimal
digit

\h

any horizontal whitespace character
(since PHP 5.2.4)

\H

any character that is not a horizontal
whitespace character (since PHP 5.2.4)

\s

any whitespace character

\S

any character that is not a whitespace
character

\v

any vertical whitespace character (since
PHP 5.2.4)

\V

any character that is not a vertical
whitespace character (since PHP 5.2.4)

\w

any “word” character

\W

any “non-word” character

Each pair of escape sequences partitions the
complete set of characters into two disjoint sets. Any given
character matches one, and only one, of each pair.

The “whitespace” characters are HT (9), LF (10), FF
(12), CR (13), and space (32). However, if locale-specific matching
is happening, characters with code points in the range 128-255 may
also be considered as whitespace characters, for instance, NBSP
(A0).

A “word” character is any letter or digit or the
underscore character, that is, any character which can be part of a
Perl “word“. The definition of letters
and digits is controlled by PCRE’s character tables, and may vary
if locale-specific matching is taking place. For example, in the
“fr” (French) locale, some character codes greater than 128 are
used for accented letters, and these are matched by
\w.

These character type sequences can appear both
inside and outside character classes. They each match one character
of the appropriate type. If the current matching point is at the
end of the subject string, all of them fail, since there is no
character to match.

The fourth use of backslash is for certain simple
assertions. An assertion specifies a condition that has to be met
at a particular point in a match, without consuming any characters
from the subject string. The use of subpatterns for more
complicated assertions is described below. The backslashed
assertions are

\b

word boundary

\B

not a word boundary

\A

start of subject (independent of
multiline mode)

\Z

end of subject or newline at end
(independent of multiline mode)

\z

end of subject (independent of multiline
mode)

\G

first matching position in
subject

These assertions may not appear in character
classes (but note that “\b” has a different meaning,
namely the backspace character, inside a character class).

A word boundary is a position in the subject string
where the current character and the previous character do not both
match \w or \W (i.e. one matches \w and
the other matches \W), or the start or end of the string
if the first or last character matches \w,
respectively.

The \A, \Z, and \z
assertions differ from the traditional circumflex and dollar
(described in anchors ) in that they only ever match at the very start
and end of the subject string, whatever options are set. They are
not affected by the PCRE_MULTILINE or PCRE_DOLLAR_ENDONLY options. The difference between
\Z and \z is that \Z matches before a
newline that is the last character of the string as well as at the
end of the string, whereas \z matches only at the end.

The \G assertion is true only when the
current matching position is at the start point of the match, as
specified by the offset argument of
preg_match(). It differs from \A
when the value of offset is
non-zero.

\Q and \E can be used to ignore
regexp metacharacters in the pattern. For example:
\w+\Q.$.\E$ will match one or more word characters,
followed by literals .$. and anchored at the end of the
string.

\K can be used to reset the match start
since PHP 5.2.4. For example, the pattern foo\Kbar matches
“foobar”, but reports that it has matched “bar”. The use of
\K does not interfere with the setting of captured
substrings. For example, when the pattern (foo)\Kbar
matches “foobar”, the first substring is still set to “foo”.