Regular expressions

Regular expressions allow you to formulate patterns to search for. Here's an example: It is easy to search for the string "Sep" in a file, you do it with

grep "Sep" file

This gives you all lines containing the string "Sep". But what do you do if you only want lines starting with "Sep", for example, to read all lines in your syslog regarding september? Then you need regular expressions. It works like this:

grep -E "^Sep" /var/log/messages

gives you all entries for september in your syslog. And there is much more you can do with regular expressions.

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"

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 Lnux 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 *.

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

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.

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