Jul 5, 2014

Extended Pattern Matching

Bash supports extended pattern matching. By using the built-in utility we can check if it is enabled or not:

$ shopt extglob

If not, to enable it:

$ shopt -s extglob

By default extglob is on in interactive shells, but off in non-interactive shells.

The key about extended pattern matching is pattern list via | (what we usually see as a OR operator). But do not think about it as that, think about it as a list of array pattern that are separated by | instead of ,. And actually one of them is the same as {} or brace expansion, but it can do more than expanding.

Here are the pattern operators:

? * + @ !

Create some example files:

$ touch a{,1,2,11,12}.js && ls
a11.js  a12.js  a1.js  a2.js  a.js

?(pattern-list)

Zero or one (any one) occurrence of the giving pattern:

$ ls a?(2|1).js
a1.js  a2.js  a.js

*(pattern-list)

Zero or more occurrences of the giving pattern (essentially everything):

$ ls a*(2|1).js
a11.js  a12.js  a1.js  a2.js  a.js

+(pattern-list)

One or more of the giving pattern (notice a.js is missing):

$ ls a+(2|1).js
a11.js  a12.js  a1.js  a2.js

@(pattern-list)

Any one of giving pattern:

$ ls a@(2|1).js
a1.js  a2.js

!(pattern-list)

None of the giving pattern:

$ ls a!(2|1).js
a11.js  a12.js  a.js

One mistake I had was getting confused between extended pattern matching and brace expansion, for example:

$ ls test/@{src|spec}/*.js
ls: cannot access test/@src/*.js: No such file or directory
ls: cannot access test/@spec/*.js: No such file or directory

Extended pattern uses parentheses () not braces {} as in brace expansion.

Also, these two patterns are the same:

test/@(src|spec)/*.js
test/{src,spec}/*.js

In some situations, extended pattern matching does not work, for example, matching files from the current directory and from one of the subdirectories with the following directory structure:

$ tree
.
├── app.js
├── lib
│   └── util.js
└── test
    └── main.js

2 directories, 3 files

I would like to match js files from the current directory and lib/ directory, sort of like:

$ ls *.js lib/*.js
app.js  lib/util.js

But this does not do it:

$ ls @(.|lib)/*.js

Instead, use brace expansion:

$ ls {.,lib}/*.js

Finally, Node’s Minimatch supports brace expansion, extended globbing and globstar.