[chm]

What does something like \D{2}\S{3} mean? Out of context it seems a bit... cryptic ;)

[comex]

It's a regular expression, probably Perl syntax:

http://perldoc.perl.org/perlrebackslash.html

[chm]

Thank you. I thought it could be a regex, but I had never seen perl syntax before.

[JoshTheGeek]

Two non-digits followed by three non-whitespace characters

[cperciva]

It's a regex... but my first thought was that it was a clue about group theory.