Look at Expression library and choose user name and/or password regex for you. You can also test your regex in online regex testers like RegexPlanet.
My regex suggestions are:
^[a-zA-Z][a-zA-Z0-9._\-]{5,}$
This regex accepts user names with minimum 6 characters, starting with a letter and containing only letters, numbers and ".","-","_" characters.
Next one:
^[a-zA-Z0-9._\\-]{6,}$
Similar to above, but accepts ".", "-", "_" and 0-9 to be first characters too.
If you want to validate only string length (minimum 6 characters), this simple regex below will be enough:
^.{6,}$