Cisco Router IOS XR Getting Started Manual page 197

Appendix A Understanding Regular Expressions, Special Characters, and Patterns
Parentheses Used for Pattern Recall
To create a regular expression that recalls a previous pattern, use parentheses to indicate memory of a
specific pattern and a backslash (\) followed by a digit to reuse the remembered pattern. The digit
specifies the occurrence of a parenthesis in the regular expression pattern. When there is more than one
remembered pattern in the regular expression, \1 indicates the first remembered pattern, \2 indicates the
second remembered pattern, and so on.
The following regular expression uses parentheses for recall:
a(.)bc(.)\1\2
This regular expression matches an a followed by any character (call it character number 1), followed by
bc followed by any character (character number 2), followed by character number 1 again, followed by
character number 2 again. So, the regular expression can match aZbcTZT. The software remembers that
character number 1 is Z and character number 2 is T, and then uses Z and T again later in the regular
expression.
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