Bash Patterns

External

• Gnu bash Manual - Pattern Matching

Internal

• bash
• bash Parameter and Variable Expansion
• Regular Expressions

Overview

Bash patterns are a simple regular expression language defined by the metacharacters and rules described below.

Variable expansion can be used inside patterns:

a="something"
b="some"
echo ${a#${b}}

prints "thing".

Metacharacters

The following characters have a special meaning when a bash pattern is evaluated, and they need to be escaped to be matched literally:

*

* (star) matches any string, including the null string. When the globstar shell option is enabled, and ‘*’ is used in a filename expansion context, two adjacent ‘*’s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a ‘/’, two adjacent ‘*’s will match only directories and subdirectories.

?

? (question mark) matches a single character. The dot ('.') is not a metacharacter.

[...]

Matches any one of the enclosed characters.

For example:

[- :]

matches '-', space and ':'.

A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale’s collating sequence and character set, is matched. A digit is matches as follows:

[0-9]

A ‘-’ may be matched by including it as the first or last character in the set.

A ‘]’ may be matched by including it as the first character in the set.

Negation

If the first character following the ‘[’ is a ‘!’ or a ‘^’ then any character not enclosed is matched. The match is for one character only.

To match a known number of characters, repeat the [!...] expression the known number of times. [!...]* does not work to match an undefined number of not-enclosed characters; it will match just one non-enclosed character then all the characters. This behavior is different than sed behavior on negation.

Classes

Within ‘[’ and ‘]’, character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX standard:

• alnum
• alpha
• ascii
• blank
• cntrl
• digit
• graph
• lower
• print
• punct
• space
• upper
• word: matches letters, digits, and the character ‘_’.
• xdigit

A character class matches any character belonging to that class.

/

To match a '/' (slash), use '\/'. If used in replacement constructs as variable expansion replacement, there is no need to escape slashes in the replacement string.

~

~ (tilda)

'

' (single quote) must be escaped to match:

\'

"

" (double quote) must be escaped to match:

\"

;

(semicolon) must be escaped to match

\;

(...)

Parentheses must be escaped to match

\(
\)

Non-Special Characters

These characters do not need to be escaped in bash patterns to match:

\ # forward slash
. # dot - does NOT match any character, it just matches a dot
: # column

Paths

See / above.

Pattern Lists

?(pattern-list)

Matches zero or one occurrence of the given patterns.

*(pattern-list)

Matches zero or more occurrences of the given patterns.

+(pattern-list)

Matches one or more occurrences of the given patterns.

@(pattern-list)

Matches one of the given patterns.

!(pattern-list)

Matches anything except one of the given patterns.