Extended pattern matching in BASH

By chys on February 26th, 2009

Many features provided by BASH are not widely known or used, but they really can be useful. One example is extglob (extended pattern matching) – with this, a pattern can be almost as powerful as a regular expression.

Use “shopt -s extglob” to enable this feature. After that, in addition to the standard asterisks, question marks and square brackets, we can also use the following five sub-patterns:

?(pattern-list): Matches empty or one of the patterns
*(pattern-list): Matches empty or any number of occurrences of the patterns
+(pattern-list): Matches at least one occurrences of the patterns
@(pattern-list): Matches exactly one of the patterns
!(pattern-list): Matches anything EXCEPT any of the patterns

The pattern-list represents one or more patterns, which can again contain these extended sub-patterns, delimited by pipe signs (|). Two simple examples:

rm -rf !(lost+found)
Removes everything except lost+found
for x in *.@(jp?(e)g|gif|png)
Loops through all files having extension jpg, jpeg, gif, or png

The following example is a little more complicated. It prints a list of default/GNU/Intel C/C++ compilers present in directories specified by $PATH:

#!/bin/bash
shopt -s extglob nullglob
x="${PATH//:/,}"
eval "printf '%s\n' {$x}/@(?([ig])cc|[cg]++|icpc)?(-+([0-9])+(\.+([0-9])))"

(NOTE: nullglob makes a pattern matching no file to expand to nothing instead of unchanged.)

Doesn’t it look like a regular expression? The output is like this in my system:

/usr/bin/c++
/usr/bin/c++-4.2.4
/usr/bin/c++-4.3.3
/usr/bin/cc
/usr/bin/g++
/usr/bin/g++-4.2.4
/usr/bin/g++-4.3.3
/usr/bin/gcc
/usr/bin/gcc-4.2.4
/usr/bin/gcc-4.3.3
/usr/x86_64-pc-linux-gnu/gcc-bin/4.2.4/c++
/usr/x86_64-pc-linux-gnu/gcc-bin/4.2.4/g++
/usr/x86_64-pc-linux-gnu/gcc-bin/4.2.4/gcc
/opt/intel/cce/10.1.018/bin/icc
/opt/intel/cce/10.1.018/bin/icpc

Unfortunately, we cannot use the following codes:

x="${PATH//:/|}"
printf '%s\n' $x/@(?([ig])cc|[cg]++|icpc)?(-+([0-9])+(\.+([0-9])))

This is not surprising, however. No sub-patterns is allowed to expand to a string including forward slashes (path delimiter)^[1]. (This means a single asterisk won’t expand to a file in a subdirectory, which is usually desired. Bash 4 has introduced ** which matches slashes as well.)

[1] In a case statement or a [[ ]] builtin (using the == operator), sub-patterns indeed match slashes.