(grep-regexp-alist): Use backreference at the
end of first regexp to limit the match to the position between line number and source line with same separator character as used between file name and line number. In the second regexp limit mouse-face area to file name and line number by adding new group for them and referring it in HYPERLINK arg. (grep-regexp-alist, grep-mode-font-lock-keywords): Use shy group. (grep-mode): Set font-lock-lines-before to 0 to not refontify the previous line where grep markers may be already removed.
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