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I am trying to understand the difference between grep -e and grep -E. Now from grep manpage I got:
-E, --extended-regexp
Interpret PATTERN as an extended regular expression (see below).
-e PATTERN, --regexp=PATTERN
Use PATTERN as the pattern; useful to protect patterns beginning with -
The above explanation does not make sense for me.
So, can someone explain it to me using examples what is the difference between the two and when to use which option.
PS: Version: grep (GNU grep) 2.10
-e is strictly the flag for indicating the pattern you want to match against. -E controls whether you need to escape certain special characters.
man grep explains -E it a bit more:
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their
special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).
Traditional egrep did not support the { meta-character, and some egrep
implementations support \{ instead, so portable scripts should avoid { in grep -E
patterns and should use [{] to match a literal {.
GNU grep -E attempts to support traditional usage by assuming that { is not
special if it would be the start of an invalid interval specification.
For example, the command grep -E '{1' searches for the two-character string {1
instead of reporting a syntax error in the regular expression. POSIX.2 allows
this behavior as an extension, but portable scripts should avoid it.
echo "hello" | grep -o -e 'h|l' I get no output because | lost its special meaning and when I do echo "hello" | grep -o -E 'h|l' I get the desired output.
echo "hello" | grep -o -e 'h' -e 'l' will work if you want to toss the -E away in this case.
grep -e -o 'h|l' will match literally h|l while grep -e -o 'h\|l' will match h or l and with -E it's the contrary
Oct 10, 2012 at 19:24
Also grep -e allows to use several strings for searching: 'grep -e 'abc' -e 'def' -e '123' will look for any of the three of these strings: abc as well as def and 123.
This works quite similar to grep 'abc\|def\|123' where \| stands for or but could be a bit clearer to read.
As the most important facts on grep -E are already explained here, I just want to add what I summed up on this topic on a quite similar question: Regular Expression for finding double characters in Bash
Just to elaborate on the -e option. -e is often optional:
grep PATTERN
is identical to
grep -e PATTERN
unless, as stated in an earlier Answer and in the man pages, there are multiple search patterns, or to protect a pattern beginning with a hyphen (-).
see below
/extended
grep understands three different versions of regular expression syntax: “basic,” “extended” and “perl.” In GNU grep, there is no difference in available functionality between basic and extended syntaxes. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards. Perl regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but may not be available on every system.
So, once again.
In GNU grep, there is no difference in available functionality between basic and extended syntaxes
(, ), | etc need to be backslash-escaped to have their special meaning in basic regexp, but not in extended (where they need to be escaped to be treated as a string literal)
-eis really just to disambiguate when a regex starts with a dash. Sogrep ---foogivesunrecognized option: ---foobut you can saygrep -e ---footo grep for the regular expression---foo.