Why javascript string match includes undefined

Question

I have a regex that is more or less used like this:

\'(801) 555-1234\'.match(/^(1[-. ]?)?\\(?[0-9]{3}\\)?[-. ]?[0-9]{3}[-. ]?[0-9]{4}$/)

Fo

Answer 1

It's because your regex starts with that parenthesized group. The undefined in the result means that nothing matched that part.

When you add the "g" suffix, the behavior of the regex code changes a little, so the return value is different. The "g" ("global") suffix causes the routine to return all the matches of the whole regex; the groups are effectively ignored in that case. For example:

"hello world! nice day today!".match(/\w+/g)

would return an array like this:

["hello", "world", "nice", "day", "today"]