Cisco Systems Cisco IOS XR manual Pattern Alternation, Anchor Characters, Underscore Wildcard

Appendix A Understanding Regular Expressions, Special Characters, and Patterns

Pattern Alternation

The order for matches using multipliers (*, +, and ?) is to put the longest construct first. Nested constructs are matched from outside to inside. Concatenated constructs are matched beginning at the left side of the construct. Thus, the regular expression matches A9b3, but not 9Ab3 because the letters are specified before the numbers.

Pattern Alternation

Alternation can be used to specify alternative patterns to match against a string. Separate the alternative patterns with a vertical bar (). Only one of the alternatives can match the string. For example, the regular expression codextelebit matches the string codex or the string telebit, but not both codex and telebit.

Anchor Characters

Anchoring can be used to match a regular expression pattern against the beginning or end of the string. Regular expressions can be anchored to a portion of the string using the special characters shown in Table A-3.

Table A-3	Special Characters Used for Anchoring
Character		Description
^		Matches the beginning of the string.
$		Matches the end of the string.

For example, the regular expression ^con matches any string that starts with con, and sole$ matches any string that ends with sole.

In addition to indicating the beginning of a string, the ^ can be used to indicate the logical function “not” when used in a bracketed range. For example, the expression [^abcd] indicates a range that matches any single letter, as long as it is not the letters a, b, c, and d.

Underscore Wildcard

Use the underscore to match the beginning of a string (^), the end of a string ($), parentheses (( )) , space ( ), braces ({}), comma (,), and underscore (_). The underscore can be used to specify that a pattern exists anywhere in the string. For example, _1300_ matches any string that has 1300 somewhere in the string and is preceded by or followed by a space, brace, comma, or underscore. Although _1300_ matches the regular expression {1300_, it does not match the regular expressions 21300 and 13000t.

The underscore can replace long regular expression lists. For example, instead of specifying ^1300( ) ( )1300$ {1300, ,1300, {1300} ,1300, (1300, simply specify _1300_.

Parentheses Used for Pattern Recall

Use parentheses with multiple-character regular expressions to multiply the occurrence of a pattern. The Cisco IOS XR software can remember a pattern for use elsewhere in the regular expression.

Cisco IOS XR Getting Started Guide

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