Cisco Systems Cisco IOS XR manual Bc.\1\2

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.

Cisco IOS XR Getting Started Guide

OL-10957-02 A-5