$ ./genex.pl 
regular__expression
regular__exp
regular__exression
regular__ex
regular_expression
regular_expression!
regular_exp
regular_exp!
regular_exression
regular_exression!
regular_ex
regular_ex!
regexpression
regexpression!
regexpression!!
regexp
regexp!
regexp!!
regexression
regexression!
regexression!!
regex
regex!
regex!!