Regex with all optional parts but at least one required

Question

I need to write a regex that matches strings like \"abc\", \"ab\", \"ac\", \"bc\", \"a\", \"b\", \"c\". Order is important and it shouldn\'t match multiple appearances of th

Answer 1

^(?=.)a?b?c?$

This will check if there is at least one character with lookahead and will match your regex.

Answer 2

To do this with pure regex you're going to have to expand it into all of its possibilities:

ab?c?|a?bc?|a?b?c

If you have lookaheads you can make sure the string is non-empty. Or you can verify the string has a length of at least one before passing it to the expression, depending on your choice of language.

For example a .NET lookahead might look like this:

^(?=[abc])a?b?c?$

Or you could just test your string's length before matching it:

if (YourString.Length == 1) {
   // matching code goes here, using the expression a?b?c?
}

Answer 3

It is pointless to try to pack all functionality of all problems you ever have into one single regexp. The best solution is to write the obvious regex, and add a check against zero length. You should only get extra clever with regexps when you absolutely have to - for instance if you have to interact with an unchangeable API that accepts only one regexp and nothing more.

Answer 4

You can write down all permutations (which is a pain) or all possibilities of which letter is not left out (ab?c?|a?bc?|a?b?c), which is somewhat less of a pain.