Alternation

Alternation allows matching one of several phrases. This is more powerful than character classes, which are limited to characters.

Delimit the set of phrases with pipes—|.

/foo|bar|baz/g

One of foo, bar, and baz

If only a part of the regex is to be “alternated”, wrap that part with a group—capturing or non-capturing.

/Try (foo|bar|baz)/g

Try followed by one of foo, bar, and baz

Matching numbers between 100 and 250:

/1\d\d|2[0-4]\d|250/g

Examples

Let’s improve one of our older examples to also match shorthand hex colours.

/#([0-9A-F]{6}|[0-9A-F]{3})/g

It is important that [0-9A-F]{6} comes before [0-9A-F]{3}. Else:

/#([0-9A-F]{3}|[0-9A-F]{6})/g

Tip

Regex engines try alternatives from the left to the right.

/^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/g