#! /bin/sh
#
# fullgrep -- Print all occurrences of a regular expression in
# a file.  Written by Hartmut Schaefer and Paolo Bonzini.
#
# The bulk of this shell script is this 3-line sed invocation:
#
#    /\n/P                  # Print matching text
#    /\n/!s/REGEXP/\n&\n/   # If not
#    D
#
# Note that the P and s/// commands are invoked alternatively: this
# is what happens after a successful substitution:
# - D deletes the skipped part of pattern space
# - P prints the first line which contains the matched text
#   because after the matched text there is a \n
# - The s/// command is skipped
# - D deletes the matched text and leaves no new-lines...
# - ...so P is skipped and s/// looks for another occurrence.
#
# In other words, the loop could also be written like this:
#
#    s/REGEXP/\n&\n/
#    /\n!/d
#    s/[^\n]*\n//           # same as D but does not restart the program
#    P
#    D
#
# Ok, this version may be more clear...
#
# Also note that we cannot match .*REGEXP because it is greedy.

SED="${SED:-sed}"
if [ "$1" = -e ]; then
  SED="$SED -r"
fi

if [ "$#" = 0 ]; then
  echo Usage: $0 [-e] REGEXP [FILE...] >&2
  exit 1
fi

regexp=$1
shift
$SED \
  -e '/\n/P' \
  -e '/\n/!s/'"$regexp"'/\
&\
/' \
  -e D ${1+"$@"}