Question

This is for validating usernames on my system.

Every username can include these symbols “.-_”, but the words separated by the mentioned symbols cannot be digits only.

An example would be

Invalid:

123.as-as123
123.AS-aS123

Valid:

a123.as-as123
A123.AS-aS123

Answer 1

I believe this will work, if I understand your requirements correctly:

/^(?:[a-z0-9]*[a-z][a-z0-9]*[.\-_])*[a-z0-9]*[a-z][a-z0-9]*$/i

It matches zero or more groups of letters/numbers ending with .,-, or _ and requiring at least one letter. Then it matches a final group of letters/numbers, requiring at least one letter.

It is case insensitive (that is what the i at the end does), so it will match uppercase as well. (This syntax works at least in Perl and PHP. In other languages, you might have to apply this option in a slightly different way, but case insensitive is always available).

One might be tempted to shorten it like this:

/^(?:[a-z0-9]*[a-z][a-z0-9]*[.\-_]?)+$/i

However, that is less desirable, because making the [.\-_] optional gives it too much flexibility in the ways it can try to match. It will probably have to run through a huge number of cases on a pattern that fails.