Execute sed substitution expression over files in a git tree

git-sed.sh

#!/bin/sh

expr="$1"
shift

pattern="$(printf "%s" "$expr" | sed -n 's/^s\/\(\([^\\\/]\|\\.\)*\)\/\([^\\\/]\|\\.\)*\/g\?$/\1/p')"

if [ -z "$pattern" ]
then
    printf "usage: git sed s/lhs/rhs/[g] [file...]\n" > /dev/stderr
    exit 2
fi

git grep -I -l -z "$pattern" "$@" | xargs -0 sed -i "$expr"