Regular Expression to allow an empty string or a string with specific length

Question

Hi I am having trouble trying to create a regex that will allow me to have the field empty or a string with length 9 if populated.

The current regex im using is: ^[0-9]{9}

Which works if the field is mandatory, however we have a new requirement that this can also be empty. I've tried ^[0-9]{0|9} but obviously that doesnt work.

Thanks in advance

Just make a normal string comparison to test whether the value is empty. No need for a regular expression here. — Felix Kling
– Felix Kling, Commented Apr 18, 2012 at 13:06
@FelixKling That requires JS, but doesn't work with <input pattern= — user123444555621
– user123444555621, Commented Apr 18, 2012 at 13:15
@Pumbaa80: Why do you assume that the expression is used in the context of JavaScript or even HTML? There is no indication of that in the question. — Felix Kling
– Felix Kling, Commented Apr 18, 2012 at 13:19
Right. I was trying to say that it may be impossible to test for an empty string by other means than the RegEx. — user123444555621
– user123444555621, Commented Apr 18, 2012 at 13:31

jensgram · Accepted Answer · 2012-04-18 13:05:14Z

7

^(|[0-9]{9})$

Should do the trick.

Edit: But seriously; do what @Felix Kling suggests in a comment: test the string via a .length property, .equals("") or the like.

answered Apr 18, 2012 at 13:05

jensgram

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2 Comments

Redseven Over a year ago

Great, thanks Jensgram. Not sure if it is because of the application im developing for but it doesn't allow me to use that expression. However tweeking your regex a bit it worked. Cheers! ^(|\d{9})$

jensgram Over a year ago

@Redseven Great. Didn't realize this was related to the HTML5 pattern attribute.

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