• PCRE regex
  • Escape sequences

  • Escape sequences
  • Escape sequences

    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

    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 “\\”.


    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:

    alarm, that is, the BEL character (hex
    “control-x”, where x is any
    escape (hex 1B)
    formfeed (hex 0C)
    newline (hex 0A)
    a character with the xx property, see
    for more info
    a character without the xx property, see
    for more info
    carriage return (hex 0D)
    line break: matches \n, \r and
    tab (hex 09)
    character with hex code hh
    character with octal code ddd, or

    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

    is another way of writing a
    is the same, provided there are fewer
    than 40 previous capturing subpatterns
    is always a back reference
    might be a back reference, or another way
    of writing a tab
    is always a tab
    is a tab followed by the character
    is the character with octal code 113
    (since there can be no more than 99 back references)
    is a byte consisting entirely of 1
    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:

    any decimal digit
    any character that is not a decimal
    any horizontal whitespace character
    (since PHP 5.2.4)
    any character that is not a horizontal
    whitespace character (since PHP 5.2.4)
    any whitespace character
    any character that is not a whitespace
    any vertical whitespace character (since
    PHP 5.2.4)
    any character that is not a vertical
    whitespace character (since PHP 5.2.4)
    any “word” character
    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

    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

    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

    word boundary
    not a word boundary
    start of subject (independent of
    multiline mode)
    end of subject or newline at end
    (independent of multiline mode)
    end of subject (independent of multiline
    first matching position in

    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,

    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

    \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

    \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”.