syntax
Back references
Back references
Back references
Outside a character class, a backslash followed by
a digit greater than 0 (and possibly further digits) is a back
reference to a capturing subpattern earlier (i.e. to its left) in
the pattern, provided there have been that many previous capturing
left parentheses.
However, if the decimal number following the
backslash is less than 10, it is always taken as a back reference,
and causes an error only if there are not that many capturing left
parentheses in the entire pattern. In other words, the parentheses
that are referenced need not be to the left of the reference for
numbers less than 10. A “forward back reference” can make sense
when a repetition is involved and the subpattern to the right has
participated in an earlier iteration. See the section entitled
“Backslash” above for further details of the handling of digits
following a backslash.
A back reference matches whatever actually matched
the capturing subpattern in the current subject string, rather than
anything matching the subpattern itself. So the pattern
(sens|respons)e and \1ibility matches “sense and
sensibility” and “response and responsibility”, but not “sense and
responsibility”. If case-sensitive (caseful) matching is in force
at the time of the back reference, then the case of letters is
relevant. For example, ((?i)rah)\s+\1 matches “rah rah”
and “RAH RAH”, but not “RAH rah”, even though the original
capturing subpattern is matched case-insensitively
(caselessly).
There may be more than one back reference to the
same subpattern. If a subpattern has not actually been used in a
particular match, then any back references to it always fail. For
example, the pattern (a|(bc))\2 always fails if it starts
to match “a” rather than “bc”. Because there may be up to 99 back
references, all digits following the backslash are taken as part of
a potential back reference number. If the pattern continues with a
digit character, then some delimiter must be used to terminate the
back reference. If the PCRE_EXTENDED option is set, this can be whitespace.
Otherwise an empty comment can be used.
A back reference that occurs inside the parentheses
to which it refers fails when the subpattern is first used, so, for
example, (a\1) never matches. However, such references can be
useful inside repeated subpatterns. For example, the pattern
(a|b\1)+ matches any number of “a”s and also “aba”,
“ababba” etc. At each iteration of the subpattern, the back
reference matches the character string corresponding to the
previous iteration. In order for this to work, the pattern must be
such that the first iteration does not need to match the back
reference. This can be done using alternation, as in the example
above, or by a quantifier with a minimum of zero.
As of PHP 5.2.2, the \g escape sequence
can be used for absolute and relative referencing of subpatterns.
This escape sequence must be followed by an unsigned number or a
negative number, optionally enclosed in braces. The sequences
\1, \g1 and \g{1} are synonymous with
one another. The use of this pattern with an unsigned number can
help remove the ambiguity inherent when using digits following a
backslash. The sequence helps to distinguish back references from
octal characters and also makes it easier to have a back reference
followed by a literal number, e.g. \g{2}1.
The use of the \g sequence with a negative
number signifies a relative reference. For example,
(foo)(bar)\g{-1} would match the sequence “foobarbar” and
(foo)(bar)\g{-2} matches “foobarfoo”. This can be useful
in long patterns as an alternative to keeping track of the number
of subpatterns in order to reference a specific previous
subpattern.
Back references to the named subpatterns can be
achieved by (?P=name) or, since PHP 5.2.2, also by
\k<name> or \k’name’. Additionally PHP
5.2.4 added support for \k{name} and \g{name},
and PHP 5.2.7 for \g<name> and
\g’name’.