My favorite regex is the following,
/^1?$|^(11+?)\1+$/
Which finds prime numbers. Although, I can't for the life of me think of a reason for using it.
http://stackoverflow.com/questions/3296050/how-does-this-reg...
/^1?$|^(11+?)\1+$/
Which finds prime numbers. Although, I can't for the life of me think of a reason for using it.
http://stackoverflow.com/questions/3296050/how-does-this-reg...
http://news.ycombinator.com/item?id=1486502
full comment thread here http://news.ycombinator.com/item?id=1486158
I suppose I could accept "regex" as not being a regular expression as such, but the two are used so interchangeably that maintaining a distinction isn't very realistic. I'd personally rather a regular expression described a regular language, and "PCRE" (or so) used for the Turing-complete expressions with a similar syntax.
So I'd argue that code is RegEx.
I guess it's just a matter of perspective though.
The first part (^1?$) allows "" and "1" to match (so that 1 is not detected as a prime).
The second part matches groups of two or more ones (11+?), repeated twice or more, ie products n*m, n ≥ 2, m ≥ 2.
The backreference means that \1 should match the exact same string as the first (11+?). It's different from using (11+?){2,} which would match n_1+n_2+n_3..., n_1 ≥ 2, n_2 ≥ 2, n_3 ≥ 2 (where submatch is independent).