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TC-199

Cisco IOS Terminal Services Configuration Guide

78-12093-02

Regular Expressions

This appendix explains regular expressions and how to use them in Cisco IOS software commands. It

also provides details for composing regular expressions. This appendix has the following sections:

• General Concepts About Regular Expressions

• Cisco Regular Expression Pattern Matching Characters

• Single-Character Patterns

• Multiple-Character Patterns

• Multipliers

• Alternation

• Anchoring

• Parentheses for Recall

• Regular Expression Examples

General Concepts About Regular Expressions

A regular expression is entered as part of a command and is a pattern made up of symbols, letters, and

numbers that represent an input string for matching (or sometimes not matching). Matching the string to

the specified pattern is called pattern matching.

Pattern matching either succeeds or fails. If a regular expression can match two different parts of an input

string, it will match the earliest part first.

Cisco configurations uses regular expression pattern matching in several implementations. The

following is a list of some of these implementations:

• BGP IP AS-path and X.29 access lists

• Modem (or chat) and system scripts

• X.25 route substitute destination feature

• Protocol translation ruleset scripts

Regular Expressions

Cisco Regular Expression Pattern Matching Characters

TC-200

Cisco IOS Terminal Services Configuration Guide

78-12093-02

Cisco Regular Expression Pattern Matching Characters

Table 25 summarizes the basic Cisco IOS regular expression characters and their functions.

Table 25 Cisco Regular Expression Characters

Regular

Expression

Character Function Examples

. Matches any single character. 0.0 matches 0x0 and 020

t..t matches strings such as test, text,

and tart

\ Matches the character following the backslash.

Also matches (escapes) special characters.

172\.1\.. matches 172.1.10.10 but not

172.12.0.0

\. allows a period to be matched as a

period

[ ] Matches the characters or a range of characters

separated by a hyphen, within left and right

square brackets.

[02468a-z] matches 0, 4, and w, but not

1, 9, or K

^ Matches the character or null string at the

beginning of an input string.

^123 matches 1234, but not 01234

? Matches zero or one occurrence of the pattern.

(Precede the question mark with Ctrl-V

sequence to prevent it from being interpreted as

a help command.)

ba?b matches bb and bab

$ Matches the character or null string at the end of

an input string.

123$ matches 0123, but not 1234

* Matches zero or more sequences of the character

preceding the asterisk. Also acts as a wildcard

for matching any number of characters.

5* matches any occurrence of the

number 5 including none

18\..* matches the characters 18. and

any characters that follow 18.

+ Matches one or more sequences of the character

preceding the plus sign.

8+ requires there to be at least one

number 8 in the string to be matched

() [] Nest characters for matching. Separate

endpoints of a range with a dash (-).

(17)* matches any number of the

two-character string 17

([A-Za-z][0-9])+ matches one or more

instances of letter-digit pairs: b8 and

W4, as examples

Regular Expressions

Single-Character Patterns

TC-201

Cisco IOS Terminal Services Configuration Guide

78-12093-02

The order for matching using the * or + character is longest construct first. Nested constructs are

matched from the outside in. Concatenated constructs are matched beginning at the left side. If a regular

expression can match two different parts of an input string, it will match the earliest part first.

Single-Character Patterns

The simplest regular expression is a single character that matches itself in the input string. For example,

the single-character regular expression 3 matches a corresponding 3 in the input string. You can use any

letter (A to Z, a to z) or number (0 to 9) as a single-character pattern. You can use also use a keyboard

character other than a letter or a number, such as an exclamation point (!) or a tilde (~), as a

single-character pattern, but not the characters listed in Table 25 that have special meaning when used

in regular expressions.

To use the characters listed in Table 25 as single-character patterns, remove the special meaning by

preceding each character with a backslash (\). The following examples are single-character patterns

matching a dollar sign, an underscore, and a plus sign, respectively:

You can specify a range of single-character patterns to match against a string. For example, you can

create a regular expression that matches a string containing one of the following letters: a, e, i, o, and u.

One and only one of these characters must exist in the string for pattern matching to succeed. To specify

a range of single-character patterns, enclose the single-character patterns in square brackets ([ ]). The

order of characters within the brackets is not important. For example, [aeiou] matches any one of the five

vowels of the lowercase alphabet, while [abcdABCD] matches any one of the first four letters of the

lowercase or uppercase alphabet.

| Concatenates constructs. Matches one of the

characters or character patterns on either side of

the vertical bar.

A(B|C)D matches ABD and ACD, but

not AD, ABCD, ABBD, or ACCD

_ Replaces a long regular expression list by

matching a comma (,), left brace ({), right brace

(}), the beginning of the input string, the end of

the input string, or a space.

The characters _1300_ can match any

of the following strings:

^1300$

^1300space

space1300

{1300,

,1300,

{1300}

,1300,

Table 25 Cisco Regular Expression Characters (continued)

Regular

Expression

Character Function Examples

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