What's the difference between "grep -e" and "grep -E" [closed] - linux

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I have a file test.txt, in which there are some formatted phone numbers. I'm trying to use grep to find the lines containing a phone number.
It seems that grep -e "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txt doesn't work and gives no results. But grep -E "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txtworks. So I wonder what's the difference between these 2 options.
According to man grep:
-E, --extended-regexp
Interpret pattern as an extended regular expression (i.e. force
grep to behave as egrep).
-e pattern, --regexp=pattern
Specify a pattern used during the search of the input: an input
line is selected if it matches any of the specified patterns.
This option is most useful when multiple -e options are used to
specify multiple patterns, or when a pattern begins with a dash
(`-').
But I don't quite understand it. What is an extended regex?

Answer 1

As you mentioned, grep -E is for extended regular expressions whereas -e is for basic regular expressions. From the man page:
EDIT: As Jonathan pointed out below, grep -e "specifies that the following argument is (one of) the regular expression(s) to be matched."
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.
But man pages are pretty terse, so for further info, check out this link:
http://www.regular-expressions.info/posix.html
The part of the manpage regarding the { meta character though specifically talks about what you are seeing with respect to the difference.
grep -e "[0-9]{3}-[0-9]{3}-[0-9]{4}"
won't work because it is not treating the { character as you expect. Whereas
grep -E "[0-9]{3}-[0-9]{3}-[0-9]{4}"
does because that is the extended grep version — or the egrep version for example.

Answer 2

Here is a simple test:
$ cat file
apple is a fruit
so is orange
but onion is not
$ grep -e 'but' -e 'fruit' file #Allows you to pass multiple patterns explicitly
apple is a fruit
but onion is not
$ grep -E 'is (a|not)' file #Allows you to use extended regular expressions like ?, +, | etc
apple is a fruit
but onion is not

Answer 3

The -e option to grep simply says that the following argument is the regular expression. Thus:
grep -e 'some.*thing' -r -l .
looks for some followed by thing on a line in all the files in the current directory and all its sub-directories. The same could be achieved by:
grep -r -l 'some.*thing' .
(On Linux, the situation is confused by the behaviour of GNU getopt() which, unless you set POSIXLY_CORRECT in the environment, permutes options, so you could also run:
grep 'some.*thing' -r -l .
and get the same result. Under POSIX and other systems not using GNU getopt(), options need to precede arguments, and the grep would look for a file called -r and another called -l.)
The -E option changes the regular expressions from 'basic' to 'extended'. It can be used with -e:
grep -e "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txt
grep -E -e "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txt
The ERE option means the same regular expressions, more or less, as used to be recognized by the egrep command, which is no longer a part of POSIX (having been replaced by grep -E, and fgrep by grep -F).