The regex /(?<=[a-zA-Z])\n(?=[a-zA-Z])/ means "An newline, preceeded by /[a-zA-Z]/ and followed by /[a-zA-Z]/." The length of the match is one, starting at the newline.

Contrast with /[a-zA-Z]\n[a-zA-Z]/ which means means "A /[a-zA-Z]/, then a newline, then a /[a-zA-Z]/." The length of the match is three, starting at the char before the newline.

Regarding your capture question, there are optimization in place in some circumstances. I don't know if this is one of them. But honestly, if you need to micro-optimize that much, get familiar with the benchmarker.