home.public.snippits/home/public/snippits/grep/grep-patterns.org

Grep Patterns ... things I never knew.

This was an exercise to understand GNU grep regular expressions and options.

https://man7.org/linux/man-pages/man1/grep.1.html

#
# The normal use cases
#

# A file to grep
$ cat foo.txt
foo
foo foo
foo bar
foo bar baz
foo|bar|baz

# basic regexs (-G) by default. '|' NOT special.
$ grep 'bar|baz' foo.txt
foo|bar|baz

# same
$ grep -e 'bar|baz' foo.txt
foo|bar|baz

# basic regexps.  '\' to make '|' "special"
$ grep  'bar\|baz' foo.txt
foo bar
foo bar baz
foo|bar|baz

# extended expressions. '|' IS special.
$ grep -E 'bar|baz' foo.txt
foo bar
foo bar baz
foo|bar|baz

#
# The suprizing truth about "-e|--regexp"
#

# -e (and --regex ) specify PATTERNs, NOT regexps
#
# "-e 'bar|baz'" specifies 'bar|baz'  as a PATTERN NOT a regexp.
#
# The (non-)interpretation of patterns as regexps
# comes from the use of -G (default), -E, -F or -P
#
# In this example it is a basic regex because -G is implied.
#
$ grep -e 'bar|baz' foo.txt
foo|bar|baz


#
# newlines to separate patterns.  Who knew?
#

# per the man page:
# NAME
#        grep - print lines that match patterns
#
# SYNOPSIS
#        grep [OPTION...] PATTERNS [FILE...]
#
# DESCRIPTION
#        grep  searches  for  PATTERNS  in  each  FILE.  PATTERNS is one or more
#        patterns separated by newline characters,
#                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#

$ grep `printf 'bar\nbaz'` foo.txt
foo.txt:foo bar
foo.txt:foo bar baz
foo.txt:foo|bar|baz