# Remove everything except a-z,0-9 


tr/a-z0-9//cd;


# Print a warning if something besides a-z,0-9 is found

print "Warning ! Did you mean '$_'? " 
          if ( tr/a-z0-9//cd ) ;


# Clarification [2001-5-29]: this is just an example, 
# modify the allowed chars set at your will. 
# You may also want to lowercase the input with 'lc' or
# 'tr/A-Z/a-z/';