# and ## only can occur once in a literal string

Question

I'm looking for a regular expression that character # and ## only can occur once in a literal string.

It should match:

a#abc
a#bc##e
a##bc#e
a##e

But it should be non-compliant about:

a#a#b#c
a##bc##e
a##bc##e##d
a###e

This seems trivial to do with a simple substring match. Does it need to be a regular expression? — Álvaro González, Mar 01 '17 at 09:40

score 2 · Accepted Answer · edited Mar 01 '17 at 10:19

2

You could use the following regex, composed of an alternation of these two patterns:

^[^#]*(?:##[^#]*#?|#[^#]*(?:##)?)[^#]*$

If the regex should match strings without any #, just make the whole alternation optional :

^[^#]*(?:##[^#]*#?|#[^#]*(?:##)?)?[^#]*$

edited Mar 01 '17 at 10:19

Toby Speight

answered Mar 01 '17 at 09:28

Aaron

Good point, Thank you for your attention! here I want to supplement the filter condition. If the literal string is `null` or `#` and `##` don't occur once in this string, it also should be compliant. It should also match: – Henry Chen Mar 01 '17 at 10:19
@HenryChen the second regex I provided should work fine then, it can match empty strings and strings that contain no `#` – Aaron Mar 01 '17 at 10:31
Yes, it do make sense. Thank you again! I will adopt this idea. – Henry Chen Mar 01 '17 at 11:24

1 Answers1