Using explicitly numbered repetition instead of question mark, star and plus

Question

I've seen regex patterns that use explicitly numbered repetition instead of ?, * and +, i.e.:

Explicit            Shorthand (something){0,1}    (something)? (something){1}      (something) (something){0,}     (something)* (something){1,}     (something)+ 

The questions are:

• Are these two forms identical? What if you add possessive/reluctant modifiers?
• If they are identical, which one is more idiomatic? More readable? Simply "better"?

Answer 1

To my knowledge they are identical. I think there maybe a few engines out there that don't support the numbered syntax but I'm not sure which. I vaguely recall a question on SO a few days ago where explicit notation wouldn't work in Notepad++.

The only time I would use explicitly numbered repetition is when the repetition is greater than 1:

• Exactly two: {2}
• Two or more: {2,}
• Two to four: {2,4}

I tend to prefer these especially when the repeated pattern is more than a few characters. If you have to match 3 numbers, some people like to write: \d\d\d but I would rather write \d{3} since it emphasizes the number of repetitions involved. Furthermore, down the road if that number ever needs to change, I only need to change {3} to {n} and not re-parse the regex in my head or worry about messing it up; it requires less mental effort.

If that criteria isn't met, I prefer the shorthand. Using the "explicit" notation quickly clutters up the pattern and makes it hard to read. I've worked on a project where some developers didn't know regex too well (it's not exactly everyone's favorite topic) and I saw a lot of {1} and {0,1} occurrences. A few people would ask me to code review their pattern and that's when I would suggest changing those occurrences to shorthand notation and save space and, IMO, improve readability.

Answer 2

I can see how, if you have a regex that does a lot of bounded repetition, you might want to use the {n,m} form consistently for readability's sake. For example:

/^  abc{2,5}  xyz{0,1}  foo{3,12}  bar{1,}  $/x 

But I can't recall ever seeing such a case in real life. When I see {0,1}, {0,} or {1,} being used in a question, it's virtually always being done out of ignorance. And in the process of answering such a question, we should also suggest that they use the ?, * or + instead.

And of course, {1} is pure clutter. Some people seem to have a vague notion that it means "one and only one"--after all, it must mean something, right? Why would such a pathologically terse language support a construct that takes up a whole three characters and does nothing at all? Its only legitimate use that I know of is to isolate a backreference that's followed by a literal digit (e.g. \1{1}0), but there are other ways to do that.