in reply to What it mathches`

As other monks have suggested, this regex is a bit odd. Are the quotes supposed to be part of the pattern? The pattern does not need to be quoted inside the regex delimiters. See perlre and perlreref.

If the quotes were added in error (i.e., they are not part of the pattern), then the regex becomes

/^abc[A\b]def$/
which not only makes more sense, but also the task of predicting matching patterns becomes trivial.

If you need a hint, see Re: What it mathches` and ignore the parts about the quotes. I also wonder if \b (backspace) is in error, but without additional information speculation on intent is merely that.