Produce illegal regexes when passed \b in the inputs

[dubzzz]

Hello,

While running some property based tests against the library, see commit or runkit, I found a bug causing a crash of regexgen when we pass some very precise strings to it.

Indeed if you try to run this code locally (at least with 1.3.0), you'll have a crash at runtime:

regexgen(["\b "," "])
// SyntaxError: Invalid regular expression: /\b? /: Nothing to repeat

Indeed ? cannot be used right after \b but it should be ok with [\b]?