Regex to match a pattern or empty value

Question

I need to match a pattern where i can have an alphanumeric value of size 4 or an empty value. My current Regex

"[0-9a-z]{0|4}");

does not works for empty values.

I have tried the following two patterns but none of them works for me:

"(?:[0-9a-z]{4} )?");
"[0-9a-z]{0|4}");

I use http://xenon.stanford.edu/~xusch/regexp/ to validate my Regex but sometimes i get stuck for RegEx. Is there a way/tools that i can use to ensure i have to come here for very complex issues.

Examples i may want to match: we12, 3444, de1q, {empty} But not want to match : @$12, #12q, 1, qwe, qqqqq

No UpperCase is matching.

Can you show an example of what it is that you want to match? — sshashank124, Apr 15 '14 at 23:32
@donfuxx no. but if i have to do i simply use A-Z/a-z|0-9 as set of chars ? — CodeMonkey, Apr 15 '14 at 23:35
read this question http://stackoverflow.com/questions/15723663/using-regex-to-filter-year-of-fixed-length-0-or-4-digit — raiserle, Apr 15 '14 at 23:35

Caio Oliveira · Accepted Answer · 2020-04-28T21:22:13.450

7

Overall you could use the pattern expression|$, so it will try to match the expression or (|) the empty , and we make sure we don't have anything after that including the anchor $. Furthermore, we could enclose it with a capture group (...), so it will finally look like this:

ˆ(expresison|)$

So applying it to your need, it would end up to be like:

^([0-9a-z]{4}|)$

here is an example

EDIT:

If you want to match also uppercases, add A-Z to the pattern:

^([0-9a-zA-Z]{4}|)$

edited Apr 28 '20 at 21:22

answered Apr 15 '14 at 23:34

Caio Oliveira

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+1 Did you read my mind? Was about to write exactly the same regex, incl. posting same demo link :-P – donfuxx Apr 15 '14 at 23:36
`Warning: An empty alternative effectively makes this group optional which suggests the alternative is completely redundant` The solution was posted by other question http://stackoverflow.com/questions/15723663/using-regex-to-filter-year-of-fixed-length-0-or-4-digit `^([0-9a-zA-Z]{4})?$` **DOWNVOTE!** – raiserle Apr 15 '14 at 23:42
Yes, thats what i want.. either an empty value or a value of 4 chars long. – CodeMonkey Apr 15 '14 at 23:43
Why would you prefer using `ˆ(expr|)$` over `ˆ(expr)?$` – nomæd Apr 19 '22 at 07:55
Just to be suggestive. In fact both are absolutely the same, and i've never found in any source it is a "bad practice" (or that it is slower and we should avoid using it). So to keep the same rational from the questioner, I opted using it this way. – Caio Oliveira Apr 21 '22 at 11:02

score 0 · Answer 2 · edited May 23 '17 at 11:46

0

I suppose "empty value" means empty line in the question above. If that's the case you can use this expression:

^\s*$|[a-z0-9]{4}

which will match alphanumeric patterns of size 4 or empty lines as explained here

edited May 23 '17 at 11:46

Community

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answered Apr 15 '14 at 23:41

Fra

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Regex to match a pattern or empty value

2 Answers2