CODEWITHSUNDEEP
regexperl
user2268507
user2268507

Reputation:

Regex OR pattern not retrieving match

I have the following Regex Pattern

/string\s+(?|(x)|(ix|iv|v?i{0,3})|([a-j]))/i

When I use this with the input

String a

I don't get a match for a

If I remove the middle or condition or move this condition to earlier in the pattern, it picks it up.

http://regex101.com/r/uV3aL0/26

Would someone know what is going on here?

Thanks for your help.

Upvotes: 1

Views: 59

Answers (2)

Avinash Raj
Avinash Raj

Reputation: 174766

The reason for this behavior is the inbetween v?i{0,3} regex. If you change your regex like,

/string\s+(?|(x)|(ix|iv|v?i{1,3})|([a-j]))/i

it would work. NOrmally regex engine would take the pattern from left to right. So after string\s+, it takes (x), but there isn't a character x, so this would fail. Next it goes to (ix|iv|v?i{0,3}) and takes the first ix. But there isn't ix or iv so these also fail. Next it takes this v?i{0,3}) pattern. An optional v and an i 0 upto three times. So an inbetween empty string would be matched. So finally we got a match and the regex engine won't go for the next OR condition.

Upvotes: 1

choroba
choroba

Reputation: 241968

The problem is the following part:

v?i{0,3}

Notice that if v is missing and there's no i, it still matches. You can fix the problem by adding $ to the end of the regex. Or, optimize the Roman numerals in a different way:

/string\s+(?|(i?[xv]|v?i{1,3})|([a-j]))/i

Upvotes: 2

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