How to create a regular expression to match specific substrings inside brackets?

Question

In my Ruby on Rails app I need a regex that accepts the following values:

I am still new to regular expressions and I came up with this:

/\A[a-zA-Z._}{#-]*\z/

This works pretty well already, however it also matches strings that should not be allowed such as:

}FOO or {YYY}

Can anybody help?

Do you mean the literal letters `DD`, `MM`, `NNNNNN`? Why is `{YYY}` not allowed if you say "upper and lowercase letters" are allowed? — Wiktor Stribiżew, Jun 10 '17 at 08:04
Within rounded brackets only the above mentioned values should be allowed. Outside the rounded brackets, the four special characters and all letters are allowed. I am not even sure if that can be done using Regex. — Tintin81, Jun 10 '17 at 08:09
Well, try [`/\A(?:\{(?:DD|MM|YY|N{2,6})\}|[A-Za-z_.#-])*\z/`](http://rubular.com/r/4yWHJl9UNn) — Wiktor Stribiżew, Jun 10 '17 at 08:11
Wow, that works perfectly! Thanks a lot. Just one n00b question: What does the `?:` stand for? — Tintin81, Jun 10 '17 at 08:22
It is a non-capturing group that does not create submatches. — Wiktor Stribiżew, Jun 10 '17 at 08:30

score 4 · Accepted Answer · answered Jun 10 '17 at 08:29

You may use

/\A(?:\{(?:DD|MM|YY|N{2,6})\}|[A-Za-z_.#-])*\z/

\A - start of string anchor
(?:\{(?:DD|MM|YY|N{2,6})\}|[A-Za-z_.#-])* - a non-capturing group ((?:...) that only groups sequences of atoms and does not create submatches/subcaptures) zero or more occurrences of:
- \{(?:DD|MM|YY|N{2,6})\} - a { then either DD, or MM, YY, 2 to 6N followed with }
- | - or
- [A-Za-z_.#-] - 1 char from the set (ASCII letter, _, ., # or -)
\z - end of string.

1 Answers1