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Regex

3.8 The Back-reference Operator (\digit)

If the syntax bit RE_NO_BK_REF isn't set, then Regex recognizes back references. A back reference matches a specified preceding group. The back reference operator is represented by `\digit' anywhere after the end of a regular expression's digit-th group (see section 3.7 Grouping Operators (( ... ) or \( ... \))).

digit must be between `1' and `9'. The matcher assigns numbers 1 through 9 to the first nine groups it encounters. By using one of `\1' through `\9' after the corresponding group's close-group operator, you can match a substring identical to the one that the group does.

Back references match according to the following (in all examples below, `(' represents the open-group, `)' the close-group, `{' the open-interval and `}' the close-interval operator):

• If the group matches a substring, the back reference matches an identical substring. For example, `(a)\1' matches `aa' and `(bana)na\1bo\1' matches `bananabanabobana'. Likewise, `(.*)\1' matches any (newline-free if the syntax bit RE_DOT_NEWLINE isn't set) string that is composed of two identical halves; the `(.*)' matches the first half and the `\1' matches the second half.

• If the group matches more than once (as it might if followed by, e.g., a repetition operator), then the back reference matches the substring the group last matched. For example, `((a*)b)*\1\2' matches `aabababa'; first group 1 (the outer one) matches `aab' and group 2 (the inner one) matches `aa'. Then group 1 matches `ab' and group 2 matches `a'. So, `\1' matches `ab' and `\2' matches `a'.

• If the group doesn't participate in a match, i.e., it is part of an alternative not taken or a repetition operator allows zero repetitions of it, then the back reference makes the whole match fail. For example, `(one()|two())-and-(three\2|four\3)' matches `one-and-three' and `two-and-four', but not `one-and-four' or `two-and-three'. For example, if the pattern matches `one-and-', then its group 2 matches the empty string and its group 3 doesn't participate in the match. So, if it then matches `four', then when it tries to back reference group 3---which it will attempt to do because `\3' follows the `four'---the match will fail because group 3 didn't participate in the match.

You can use a back reference as an argument to a repetition operator. For example, `(a(b))\2*' matches `a' followed by two or more `b's. Similarly, `(a(b))\2{3}' matches `abbbb'.

If there is no preceding digit-th subexpression, the regular expression is invalid.