Add beginning-of-match anchor

[m-ender]

Add an anchor which matches at the first position of the match. This can be useful to asserting a relative position while in a lookaround. For instance, it could have saved 6 bytes in this code. It should be possible to patch this into Retina, by simple string pre-processing of the regex.

If such an anchor is present, prepend

(?=(?<MyAnchor>[\s\S]*))

to the pattern. The anchor itself can then be replaced by

(?=\k<MyAnchor>\z)

[m-ender]

Note to self, if I implement this, I have to take RTL-mode into account. In that case I would need to append

(?<=(?<MyAnchor>[\s\S]*))

And replace the anchor with

(?<=\A\k<MyAnchor>)