Regular Expressions are super powerful, and are included in most programming languages.
Regardless of what programming language you use, regular expressions will make certain text processing tasks a breeze.
If you have ever looped through every character in a string of text searching for specific characters just stop and learn regular expressions.
You will be so glad you did.

Perl

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

It is difficult to search for users (in code) based on profile information in Drupal 6 due to the flexible nature in which the profiles module stores information. Basically profiles are stored in 2 tables.

mysql> select * from profile_fields;
+-----+------------+-------------------+-------------+---------------------+------+-----------+--------+----------+----------+------------+--------------+---------+
| fid | title      | name              | explanation | category            | page | type      | weight | required | register | visibility | autocomplete | options |