(Note: Title doesn't seem to clear -- if someone can rephrase this I'm all for it!)
Given this regex: (.*_e\.txt), which matches some filenames, I need to add some other single character suffixes in addition to the e. Should I choose a character class or should I use an alternation for this? (Or does it really matter??)
That is, which of the following two seems "better", and why:
a) (.*(e|f|x)\.txt), or
b) (.*[efx]\.txt)
Use [efx] - that's exactly what character classes are designed for: to match one of the included characters. Therefore it's also the most readable and shortest solution.
I don't know if it's faster, but I would be very much surprised if it wasn't. It definitely won't be slower.
My reasoning (without ever having written a regex engine, so this is pure conjecture):
The regex token [abc] will be applied in a single step of the regex engine: "Is the next character one of a, b, or c?"
(a|b|c) however tells the regex engine to
a. If so, success. If not:b. If so, success. If not:c. If so, success. If not:Here is a benchmark:
updated according to tchrist comment, the difference is more significant
result:
Rate alternation class alternation 2855/s -- -50% class 5677/s 99% -- With a single character, it's going to have such a minimal difference that it won't matter. (unless you're doing LOTS of operations)
However, for readability (and a slight performance increase) you should be using the character class method.
For a bit further information - opening a round bracket ( causes Perl to start backtracking for that current position, which, as you don't have further matches to go against, you really don't need for your regex. A character class will not do this.