Question

(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)

Answer 1

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

• remember the current position in the string for backtracking, if necessary
• check if it’s possible to match a. If so, success. If not:
• check if it’s possible to match b. If so, success. If not:
• check if it’s possible to match c. If so, success. If not:
• give up.