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.