Introduction to the use of Linux regular expressions

Basic Introduction to Regular expressions

A regular expression uses a single string to describe and match a series of strings that conform to a certain syntactic rule. In many text edits, regular expressions are often used to retrieve and replace text that conforms to a pattern.

The basic elements of regular expressions include ordinary character and metacharacters characters, for example, A, B, 1, 2 characters are ordinary characters, ordinary characters can be understood literally, such as: A can only be understood as the English lowercase letter A, there is no other hidden meaning. While *, ^, [] and other meta-characters, the shell gives them beyond the literal meaning of meaning, such as: * The literal meaning of the symbol is only a symbol, but actually represents the repetition of the preceding character 0 or more hidden meanings. Therefore, mastering the basic elements of regular expressions is mainly to grasp the meaning of meta-characters in regular expressions. The POSIX standard divides regular expressions into two categories: a basic regular expression and an extended regular expression.

2.grep concept

The full name of grep is globalsearch regular expression (RE) Andprint out of the line, which translates to a comprehensive search for regular expressions and prints the lines. In fact, grep is not a separate program, but a family. Includes grep, Egrep, and Fgrep. The use of Egrep and fgrep is not significantly different from grep. Egrep is the extension of grep, an extended regular expression.

Use format for 3.grep

grep [OPTIONS] PATTERN [FILE ...]

The common [OPTIONS] are the following:

-I: Ignore case when matching

–color: Color display of matched strings

-V: Shows rows that are not matched by the pattern

-O: Displays only strings that are matched by the pattern

-a#: Indicates that the line below it is displayed after the matched row

-b#: Indicates that the line that precedes it is displayed after the matched line

-c#: Indicates that the # lines before and after the matching line are displayed

-E: Using extended regular expressions

Basic Regular Expressions:

The text regular_express.txt content used in the following example is reproduced on the bird's website. The contents are as follows.

"Open Source" is a good mechanismto develop programs.

Apple is my favorite food.

Football game isn't use feet only.

This dress doesn ' t fit me.

However, this dress was about $3183 dollars.

GNU is free air isn't free beer.

She hair is very beauty.

I can ' t finish the test.

Oh! The soup taste good.

Motorcycle is cheap than car.

This window is clear.

The symbol ' * ' is represented as start.

Oh! mygod!

The GD software is a library for draftingprograms.

You are the mean of the best is. 1.

The world <Happy> are the same with "glad".

I like dog.

Google is the best tools for search keyword.

Goooooogle yes!

Go! Go! Let ' s go.

# I AM Vbird

The colors that are displayed by the grep and Egrep commands used are aliases defined in the user's ~/.BASHRC directory

Alias grep= ' Grep–color '

Alias egrep= ' Egrep–color '

Metacharacters

.: Matches any single character

For example:

[Email protected]]# grep ' G.. G ' Regular_express.txt

Google is the best tools for search keyword.

*: Indicates a match to its preceding character any time

For example:

[[Email protected]]# grep ' goo*g ' regular_express.txt

Google is the best tools for search keyword.

Goooooogle yes!

. *: Any character of any length

For example:

[[Email protected]]# grep ' goo.*g ' regular_express.txt

"OpenSource" is a good mechanism to develop programs.

Google is thebest tools for search keyword.

Goooooogle yes!

\?: Match its preceding character 1 or 0 times

For example:

[[Email protected]]# grep ' gooo\?g ' regular_express.txt

Google is thebest tools for search keyword.

\+: The preceding character appears at least once

For example:

[[Email protected]]# grep ' goo\+ ' regular_express.txt

"OpenSource" is a good mechanism to develop programs.

Oh! The soup taste good.

Google is thebest tools for search keyword.

Goooooogle yes!

\{m\}: Exact Match of the preceding character is M times

For example:

[[Email protected]]# grep ' goo\{2\} ' regular_express.txt

goooooogleyes!

\{m,n\}: Matches the preceding character at least m times, up to N times

For example:

[[Email protected]]# grep ' goo\{2,5\} ' regular_express.txt

Goooooogle yes!

[]: Specify any single character within the matched range

For example:

[[Email protected]]# grep ' [HEF] ' regular_express.txt

Footballgame isn't use feet only.

However,this dress is about $3183 dollars.

Her hair isvery beauty.

TheWorld

[^]: Specify any single character outside of the matching range

For example:

[Email protected]]# tail-2 regular_express.txt|grep ' [^a-za-z] '

# I Amvbird

Here are some of the most commonly used special characters, not one for example

[[: Space:]]: Where [: space:] represents the white space character of this range, plus a [] to match the white space character

[[:p UNCT:]]: punctuation

[[: Lower:]]: lowercase letters

[[: Upper:]]: Uppercase

[[: Alpha:]]: Uppercase and lowercase letters

[[:d Igit:]]: Number

[[: Alnum:]]: Numbers and uppercase and lowercase letters

Anchoring for location:

^: Anchors the beginning of the line, indicating that any content behind the ^ must appear at the beginning of the line

For example:

[[Email protected]]# grep ' ^go ' regular_express.txt

Google isthe best tools for search keyword.

goooooogleyes!

Go! Go! Let ' s go.

$: Anchors the end of the line, indicating that any content in front must appear at the end of the row

^$: Indicates a blank line

For example:

[Email protected]]# grep ' ^$ ' regular_express.txt |wc-l

\< or \b: Anchor word, any character following it must appear as the first word

\> or \b: anchors the ending, any character in front of which must appear as the tail of the word

For example:

[[Email protected]]# grep ' ^\<root\> '/etc/passwd

Root:x:0:0:root:/root:/bin/bash

[[Email protected]]# grep ' ^\broot\b '/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Group:

\ (\): for example \ (ab\) * indicates that the match AB can appear any time, representing the match AB this combination, the main back reference)

For example:

[email protected]]# grep "\ (ab\) \{2,3\}"./fith.txt

Abababx

Ababababxy

Xyababaxbababy

Back reference: In the pattern, if you use \ (\) to implement a grouping, in a text check, if \ (\) pattern matches to something, the pattern behind this content can be referenced

\1: Indicates what appears in the 1th parenthesis

\2: Indicates what appears in the 2nd parenthesis

....

For example:

[Email protected]]# Cat Test

Helove his lover

Shelike her lover

Helike is Liker

Shelove her Liker

Helike him

[email protected]]# grep ' \ (L.. e\). *\1′test

Helove his lover

Helike is Liker

Here the parentheses are (L: e), \1 indicates that the front parenthesis appears to be love,\1, which represents love, parentheses

The like,\1 is the representative of the like

To extend the regular expression:

Extended regular expressions, most metacharacters are the same as basic regular expressions, and some are special as follows:

?: Do not need to use \, match its preceding characters 1 or 0 times

For example:

[Email protected]]# egrep ' goooo? ' Regular_express.txt

Goooooogle yes!

+: Its preceding character at least once, equivalent to the \{1,\ of the basic Regular expression}

For example:

[Email protected]]# egrep ' goo+ ' regular_express.txt

"OpenSource" is a good mechanism to develop programs.

Oh! The soup taste good.

Google is thebest tools for search keyword.

Goooooogle yes!

{M,n}: Do not need to use \, match its preceding characters at least m times, up to N times

For example:

[Email protected]]# egrep ' go{4,6} ' regular_express.txt

Goooooogle yes!

(): Group

For example:

[Email protected]]# egrep ' (oo) {3,5} ' Regular_express.txt

Goooooogle yes!

|: Or as a|b,a or B, is the entire left or right

AB|CD: Represents AB or CD

A (b|c) d

Fgrep: Regular expressions are not supported:

Any pattern that follows is used to match him as a character, it's faster, and if you don't need a regular expression search, you can use it

For example:

[Email protected]]# fgrep–color ' gooo ' regular_express.txt

goooooogleyes!

The above simple list of the regular expression of some of the use, to be proficient, but also need more practice

This article is from the "Devil of Tears" blog, please be sure to keep this source http://sepizi.blog.51cto.com/9773154/1596631

Introduction to the use of Linux regular expressions