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}.