Simple usage of regular expressions and pipeline and redirection

The regular expression (Regular EXPression), in simple terms, is the function of retrieving a string that conforms to some grammatical rules. Here we take grep as an example to learn regular expressions:

grep: Filters text commands using patterns defined by basic regular expressions;

Usage of grep:

grep [options] pattern [FILE ...]

Options

-I: Ignore character case

--color: Matching character display Color example: Alias Grep=grep--color)

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

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

-E: Using extended regular expressions

-A: Shows two lines behind the matching file

-B: Show two lines to the front of the matching file

-C: The contents of the two lines before and after the matching file are displayed

Regular expression:

Basic Regular Expressions

Extending regular Expressions

(The number of matches is in greedy mode, that is, the match is as long as possible.) ）

Basic Regular Expressions

Metacharacters

.: Match any single character

# grep ' root. ' a.txt--> shows the line containing root in the A.txt file and followed by any character behind it

*: Matches the previous character any time (can be 0 times)

A*b--> B,ab,aab,aaab

# grep ' a*b ' test.txt

. *: Match any character of any length

A.*B:AB,AAB,AMNB. --> (the file contains a line ending with a b at the beginning of a)

\?: The characters in front of the match are 1 or 0 times (partially matched on the line)

\{m,n\}: Matches the preceding character at least m times, at most n times. (modifier first \ preceding character)

# grep ' a\{1,3\}b ' test.txt--> match A at least once 3 times

[]: matches any single character within the specified range

[0-9]: Take any number in 0-9

[^]: matches any single character outside the specified range

[^[0-9]]: any one character other than 0-9

Character Set combination

[: Space:]: blank characters

[:p UNCT:]: Punctuation

[: Lower:]: Small Letter

[: Upper:]: Capital letters

[: Alpha:]: Uppercase and lowercase letters

[:d Igit:]: Digital

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

Anchor of Position:

^: Anchor the beginning of the line, any content after this character must appear at the beginning of the line

grep "^root"/etc/passwd--> line with root in/etc/passwd file

$: Anchor line end, any content before this character must appear at the end of row

bash$--> the line with bash as the end of the line

^$: Blank Line

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

\<abab--> Abab There are characters behind it, generally used to specify that a string is not at the end of a line

\> or \b: anchoring the suffix, any character preceding it must appear as the tail of the word (IBID.)

Group: \ (\)

\ (ab\) *: AB can appear any time or 0 times. (Take AB as a whole)

Back reference: The string that appears before can be called

\ (\) ... \1..\2..\3

\ n: The nth opening parenthesis and all the contents of the corresponding closing parenthesis

# grep ' ^\ ([0-9]\). *\1$ '/etc/inittab

--> in a/etc/inittab file, a line that begins with a number and ends with the same number