Need help on regular expression

[shelbytll]

Hi,

how do I write a regular expression with the following rules:
string must contain at least 20 and above of letters containing only [a-z] or [0-9] and followed by a "_" and followed by 0 or many letters.

thanks

[Weedpacket]

A suggestion to simplify the problem. Check the length of the string first, and only if it's twenty characters or longer worry about the regexp.