remove-match-lines-single.el

;; -*- coding: utf-8; lexical-binding: t -*-
;; emacs -Q --script remove-match-lines-single.el log.txt '^\n.*\n'

(defun remove-match-lines (file regex)
  (with-current-buffer (find-file-noselect file)
    (goto-char (point-min))
    (while (re-search-forward regex nil t)
      (replace-match ""))
    (save-buffer)))

(defun replace-escape-new-line (str)
    (replace-regexp-in-string "\\\\n" "\n" str))

(let ((file (car argv))
      (regex (replace-escape-new-line (nth 1 argv))))
  (remove-match-lines file regex))