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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):

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.


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