Issue parsing regex => improve docs and error message

[lukasmalkmus]

I wanted to use go-proto-validators to validate the field of a message by using regular expressions. This is the regular expression I used: [\p{L}\p{N}]({\p{L}\p{N}_- ]{0,28}[\p{L}\p{N}])?.

And... It didn't work as expected. Why? The regex isn't valid, because you can't use \ in a string ". Take a look at the generated code:

regexp.MustCompile("[\p{L}\p{N}]({\p{L}\p{N}_- ]{0,28}[\p{L}\p{N}])?.") # Does panic!

You need to escape the backtick with another one: [\\p{L}\\p{N}]({\\p{L}\\p{N}_- ]{0,28}[\\p{L}\\p{N}])?.. This works, but also produces and ugly error message.

I can't decide if this should be treated like an error or not. But we should at least put a note in the README.

[mwitkow]

Ok, this sounds like whatever we're parsing from protobuf field options should be escaped into the MustCompile string, or the MustCompile codegen should be using """. Fancy taking a stab at fixing this?

[lukasmalkmus]

@mwitkow Take a look at #36. I would love some feedback and additional testing.