[c0nstantine]

Epsilon injection appears whenever right or left side of ':' has no operand. E.g.

(:a)

(a:)

a:|b

a|b:

etc

I will try to change the precedence and see how it works. Btw what do you think about explicit operators '>' '<' where '<' works as usual regex matcher, and '>' as a generator. For example to change 'cat' to 'dog' there could be something like '<cat>dog' where '<cat' part is a parser and '>dog' is a generator. Thanks.

[kccqzy]

I think your epsilon injection rule is trying to achieve this kind of production:

    TRRE <- TRRE ':' REGEX | ':' TRRE | TRRE ':' | REGEX | ...

I think this would work better, but ':a:' is still ambiguous: it has two parse trees.