def chars_before(chars, match):
 if (match.start <= 0):
  return True
 return (match.input_string[(match.start - 1)] in chars)

def chars_before ( chars , match ) : if ( match . start <= 0 ) : return True return ( match . input_string [ ( match . start - 1 ) ] in chars )
validate the match if left character is in a given sequence .