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}$/)

For some reason this returns

["(801) 555-1234", undefined]

If I add the global flag to the regex (e.g. ...{4}$/g), the undefined value drops out and I get

["(801) 555-1234"]

I'd prefer not to use the g flag if it's not necessary (which it would seem to me it's not, since the regex begins with ^ and ends with $).

P.S. ignore the quality of the regex for it's purpose of matching phone numbers. It may not be ideal, but is from code I'm maintaining. Mostly I'm interested in the ^...$ and the presence/absence of the flag and the undefined value.

Why is undefined showing up, and why does the flag make the difference?

Ry- · Accepted Answer · 2014-01-09 16:45:33Z

10

Here is a group:

/^(1[-. ]?)?

.match (without the /g flag) and .exec return groups as part of the array. If the group didn’t match, its value is set to undefined.

Get the first element:

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

If you really, really, really want the single-element array for some reason, you can make it non-capturing:

/^(?:1[-. ]?)?

However, at that point, you have this regular expression anchored to both the start and end of the string and aren’t extracting any information. In that case, it seems like you’re really looking for RegExp.prototype.test:

var PHONE_NUMBER = /^(1[-. ]?)?\(?[0-9]{3}\)?[-. ]?[0-9]{3}[-. ]?[0-9]{4}$/;
var isValid = PHONE_NUMBER.test('(801) 555-1234');

answered Jan 9, 2014 at 16:45

Ry-♦

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2 Comments

jinglesthula Over a year ago

Ah! that makes total sense now. The array elements reflect all capturing groups (whether they were optional or not). Since the first group didn't find a match the value of that match is undefined. Makes total sense now that you say it. Looking at the MDN example I see that the array will include both matches for the regex as a whole as well as matches to capturing groups. Thanks.

user3310334 Over a year ago

"If the group didn't match, its value is set to undefined" - but I wish the designers had decided that .groups should only contain the keys of groups that actually did match.

Pointy · Accepted Answer · 2014-01-10 15:11:33Z

6

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"]

edited Jan 10, 2014 at 15:11

answered Jan 9, 2014 at 16:44

Pointy

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3 Comments

jinglesthula Over a year ago

do you know details on how the behavior changes? that would be nice to know, although it's not entirely critical to understanding where the undefined came from (which you've answered well)

Pointy Over a year ago

@jinglesthula thanks for asking - I meant to come back and add that because I needed to check the documentation to make sure I remembered correctly, but I forgot :) I'll update in a moment.

ptim Over a year ago

javascript.info/…

Niet the Dark Absol · Accepted Answer · 2014-01-09 16:44:54Z

3

You have a captured subpattern: (1[-. ]?)?

It is optional.

In this case, the option is to not match it.

Thus, it is undefined.

Try using a non-capturing subpattern: (?:1[-. ]?)?

answered Jan 9, 2014 at 16:44

Niet the Dark Absol

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Comments

Rakesh KR · Accepted Answer · 2014-01-09 17:24:13Z

In Your Regex (1[-. ]?)? indicates

NODE                     EXPLANATION
--------------------------------------------------------------------------------
  (                        group and capture to \1 (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
  1                        '1'
--------------------------------------------------------------------------------
  [-. ]?                   any character of: '-', '.', ' '
                           (optional (matching the most amount
                            possible))
--------------------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)

enter image description here

And Try (?:1[-. ]?)?

NODE                     EXPLANATION
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
   1                       '1'
--------------------------------------------------------------------------------
  [-. ]?                   any character of: '-', '.', ' '
                           (optional (matching the most amount
                           possible))
--------------------------------------------------------------------------------
  )?                       end of grouping

enter image description here

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