Regular expressions

Regular expressions allow you to formulate string patterns to search or replace. For example, to show all lines that begin with the string Sep 13 in a file myfile.txt issue:

grep -E "^Sep 13" myfile.txt

In this case ^Sep 13 is your regular expression. You used it to search for a string. And there is much more you can do with regular expressions.

RegEx cheat sheet

Escaping

The characters ^ and \ are seen as control-characters. ^ means "at the beginning of a line". With a backslash, you can escape these control-characters, meaning they act as body-characters again:

grep "^hallo" file

finds all occurrences of "hallo" at the beginning of a line in file.

grep "\^hallo" 

finds all occurrences of "^hallo" in a file

grep "\\^hallo"

finds all occurrences of "\^hallo" in a file

grep "\\\\^hallo"

finds all occurrences of "\\^hallo" in a file And so on...

Write regular expressions

For "finding a pattern defined by a regular expression", we speak of "matching".

Beginning of a line is

grep "^hallo" file

prints all occurrences of "hallo" at the beginning of a line in file.

The end of a line

grep "hallo$" file

prints all occurrences of "hallo" at the end of a line in file.

Find string1 OR string2

grep -E "Sep|Aug" file

prints all lines from file that contain "Sep" or "Aug".

Match a group of characters

grep -E "L[I,1]NUX" file

prints all lines from file that contain "LINUX" or "L1NUX"

Match a range of characters

grep -E "foo[1-9]" file

prints all lines from file that contain "foo1" or "foo2" till "foo9"

NOT the following characters

To invert matching for a group of characters

grep -E "for[^ e]" file

prints all lines from file that contain "for", but not followed by a space or an e, so not "for you" or "foresee"

Also

[^\n]*

means "all characters till the next newline". This can be useful when writing parsers.

With grep you have an additional possibility to invert matches:

grep -Ev "gettimeofday" file

prints all lines from file that do NOT contain "gettimeofday". This is a grep feature.

Any character

grep -E "L.nux" file

matches any character that is not a newline, e.g. Linux, Lenux and L7nux in file.

Match one or more times

grep -E "L[i]+nux" file

Match if i is there at least once in file The + here is a quantifier. It means, that i occurs 1 or more times. It is also possible to accept 0 or more times if you replace the + by a *.

Match n times

/etc/services is a table for protocols (services) and their port numbers. The protocols are filled up with blanks to have 16 characters. If you want to replace all protocols for port 3200 with sapdp00 you do it like this:

sed -ri "s/.{16}3200/sapdp00 3200/" /etc/services

Backreferences

Backreferences allows you to reuse matches. For example consider the following line from /var/log/apache2/access_log:

84.163.99.149 - - [21/Jan/2012:15:23:40 +0100] "GET /wiki/Special:RecentChanges HTTP/1.1" 200 66493 "http://www.linuxintro.org/index.php?title=Configuring_and_securing_sshd&action=history" "Mozilla/5.0 (X11; Linux x86_64; rv:9.0.1) Gecko/20100101 Firefox/9.0.1"

If you want to "extract the string containing GET between the quotes" you best use backreferences like this:

cat /var/log/apache2/access_log | sed "s;.*\(GET [^\"]*\).*;\1;"

Read regular expressions

*

An asterisk is a quantifier saying "whatever number of".

grep -E "Li*nux" file
Lnux
Linux
Liinux
Liiinux

An asterisk is placed next to an atom that can be repeated in whatever number. In the above example, the atom is the i character, but it can also be a group of characters:

grep -E "ba(na)*" file
ba
bana
banana
bananana

^

The ^ character stands for

• the beginning of a line if it stands at the beginning of a branch

# grep ^foo
barfoo
foo
foo

• "not" if it stands behind a bracket

# grep for[^e]
foresee
for each
for each

• the ^ character if it is escaped

# grep "\^"
adsf
as^df
as^df

?

The ? character stands for

• non-greedy matching:

http://.*?/

Understand regular expressions

Branches, Pieces and Atoms

A regular expression consists of one or more branches, separated by "|", the "OR" sign. If one of the branches matches, the expression matches:

grep -E "Tom|Harry"

Here, the expression is Tom|Harry, and Tom and Harry are both branches.

A branch consists of one or more pieces, seen in its particular order. A piece is an atom optionally followed by a quantifier:

grep -E "To*m"

Here, T is a piece as well as o* and m.

An atom is a character, a bracket expression or a subexpression. Each line can be an atom:

a
b
[^e]
(this is a subexpression)

quantifiers

A quantifier is used to define that an atom can exist several times. The * quantifier defines the atom in front of it can occur 0, 1 or several times:

grep -E "To*m"

Will find all lines containing Tom, Toom, Tooom and Tm.

See also

• scripting tutorial
• RegEx ComPoser
• RegEx training