I have written the following code to test whether string $s contains between 6 and 20 characters. Furthermore, each character must be either a letter or a number; no fancy characters, including underscores, are allowed. 

if(
 ($s =~ /\w{6,20}/)
 && ($s !~ /\_/))
{print "success!";}
else
{print "invalid string";}

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And after writing this code I just answered my own question. Sorry guys, no response needed (unless you know of a shorter version of this code).

In reply to how should i test whether a string contains between 6 and 20 letters and numbers by keiusui

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