Question

I want to know what the difference is between these two regular expressions, what are the pro’s and con’s.

Example input (date) 31-12-2012.

Method A:
/(\d{2}-\d{2}-\d{4})/

And:

Method B:
^[0-9]{2}[-/][0-9]{2}[-/][0-9]{4}\$

Answer 1

1. The first has delimiters /, the second one doesn’t. For now, I assume that to be a copy/paste issue.
2. B forces a date to occur as the first item in the string with ^, A just agrees with "a datestring anyw00-00-0000where in the string".
3. A captures the date in match 1 by the extra (), B does no such thing. As the entire match will be the 0th item in a match, you could lose the unneeded ()‘s.
4. \d vs [0-9] -> see Avner’s answer.
5. A only matches - as the day/month/year separator. Use that if you only expect -. If you expect BOTH - AND /, use [-/] as in B.
6. B wants the the date to end in $, A doesn’t. Use the one which applies. If I assume this is a copy/paste error ($ being escaped because it is in a double quoted string for no good reason), it makes B match only a date because of the ^regex$ anchoring, and A a date string anywhere in the input. Once again, use the option that applies to your data.
7. Neither of them validates a date. Only a format that kinda looks like one, but could as well not be one.