Regular expression: find range except for one letter or a range

In addition to what Kenny mentions:

1. The JDK (at least) supports this syntax:

   [a-z&&[^m-o]]

2. A couple of engines (including the .NET framework) support this:

   [a-z-[m-o]]

You should be able calculate the difference yourself.

[a-lp-z]

If the regex engine supports lookahead assertion, you could use

(?![m-o])[a-z]

but this would probably be less efficient.