added boundedLength method to Expression

GitHub issue #26 - Exception: java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length

GitHub issue #26 - Exception: java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length
added boundedness check

GitHub issue #26 - Exception: java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length
added LookBehindProps (adapted from reb4s) and added filters to group generators

GitHub issue #26 - Exception: java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length
added missing boundedLength implementations

GitHub issue #26 - Exception: java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length