RegExp matching '\n' using lookaround

Question

i have a multi-line matching case in javascript that

• only match '\n', do not consume other chars
• '\n' is not at the beginning of line (empty line)
• '\n' is not followed by '#'

seems it should be something like /(?<=.+)\n(?!#)/m
but javascript does not support lookbehind, how can i write this regex?

Answer 1

You can't. A typical trick to get lookbehind is to temporarily reverse the string and act on that, but that only works when there is no lookahead as well. See http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript for some other possibilities including one which supports lookbehind and lookahead (but not expressed as a single regex)--see the section "Mimicking lookbehind using a while loop and regexp.lastIndex".