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 *.

After reading Flip the sripct for a moment , I have to admit that there are actually-existing invisible strings which have shaped our ways of actions and responses towards different social issues. I have to accept the conception that people whoever inside a certain society, should have been restricted in a special kind of behaviors and habits in daily life. There are lots of copies accompanied with wide-ranged progressing so that the whole society is moving forward all the time. In my opinion, looking inside the ways of humans thoughts and actions, is supposed to be a complex and unexpected way which can not be obtained by general people who have no extraordinary capability of empathy. Referring to empathy, it is no wonder an uncommon ability which is full of skills to step out of own shoes and stand on others points with constant reconsidering the world. If a majority of people have possessed the way to manipulate empathy correctly, there is no wonder that less bias, wars, conflicts appearing in the world. What s more, the imbalanced definitions of different races and genders should have disappeared already. However, the reality isn t as perfect and well-behaved as we expect. The long-lasting shortage of equitable judgment and definition towards different social phenomena are really difficult and complicated to delete within a small amount of time even inside a tiny range of location. That s why human beings are struggling the unique ways which are acceptable within individuals themselves but not suitable for the entire society out because they have to compromise to the wholly social rules as long as they want to step further. Apparently, individuals are really weak so that they have no tremendous and effective influences to change anything about their own behaviors. According to this incapability, society is acting out in its own way which is though based on the summary of individuals behaviors but it turns out more like a group-thinking which should obey and perfect the upper-classes benefits. That s how the huge and diverse society has been working so well- just on the basis of the decisions and sole selfishness from a small class of politicians and riches who have totally unimaginable great powers to control almost the ways of social actions. Just considering the races without matters with gender differences, it seems ridiculous to punish any unequal former perspectives because different races built up with different cultures and wealth really have own styles and even have accepted their own social levels and divisions positively. That is to say, looking back for a really long-gone history, race bias has been defined as a natural solution to social classes divisions and people all over the world are potentially willing maybe actually they have to accept the way it is. It is the same way how gender difference works. Invisible strings establish the background of the society which has a large and wide influential impedes for the equal conception to last forever. Regarding people as fish, the society as the water we have to choose to live in, none of us can leave it for a long time though we can flee and escape the environment and regulate own styles for a while; we should confirm that we have to find out a way to come back even if we have discovered what is wrong inside the society or what should be done in eager needs. Only own to the natural fundamental rule that fishes have to live in water, we are the last and un-easiest ones to realize the conditions or stay awaken for a periods. That s the mysterious sociology, we should try to think like a sociologist though it is kinds of confusions resulting from lots of hidden surprises whatever bad or good. Never doubt the way, it is just what is should be acted.

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.

See also

• scripting tutorial
• RegEx training

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