14.3.8 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 Grouping Operators).

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.