You can have a better code sample in the test files of this library, concretely: https://github.com/yeoman/grunt-usemin/blob/master/test/test-usemin.js#L233
Where you can find the following code:
it('should allow for additional replacement patterns', function () {
grunt.file.mkdir('images');
grunt.file.write('images/image.2132.png', 'foo');
grunt.log.muted = true;
grunt.config.init();
grunt.config('usemin', {
js: 'misc.js',
options: {
assetsDirs: 'images',
patterns: {
js: [
[/referenceToImage = '([^\']+)'/, 'Replacing image']
]
}
}
});
grunt.file.copy(path.join(__dirname, 'fixtures/misc.js'), 'misc.js');
grunt.task.run('usemin');
grunt.task.start();
var changed = grunt.file.read('misc.js');
// Check replace has performed its duty
assert.ok(changed.match(/referenceToImage = 'image\.2132\.png'/));
});
I hope that helps!