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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 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):

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 bs. Similarly, (a(b))\2{3} matches abbbb.

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