Added regex validation

Added code to test if a regex is valid. This uses preg_match with a null
variable instead of string to test the regex. If this does not result in
boolean false the regex is considered valid. This does not mean the
regex will yield a result.
Is regex valid code added

Continued the check for valid regular expressions and added GUI hints to
make it more userfriendly
Merge branch 'feature-validate-regex'
Added regex valid code & changed changelog