Regex: multiple lookbehinds

Question

I'm struggling with this regex for days. I have this text:

alias -g NULL="> /dev/null 2>&1"    # redirect output
test                                # don't match
alias l="ls -lahGF"                 # list all
PROMPT=$'$FG[237]'                  # don't match

And I want to match NULL and l using grep or other terminal commands. Is it possible to directly match them without using groups?

I tried using (?<=^alias )(?<=-g).+ with no result.

Answer 1

You could use the below grep command which uses a PCRE regex.

grep -oP '^alias(?:\s+-g)?\s+\K[^=]+(?==)' file

OR

grep -oP '^alias(?:\s+-g)?\s+\K\w+' file

OR

$ grep -oP '^alias(?:\s+-g)?\s+\K[^=]+' file
NULL
l

\K discards the previously matched characters from printing at the final. \K keeps the text matched so far out of the overall regex match.