grep Regular Expression

Need for Text lookup:

Grep:global research, based on the pattern, searches for text and displays lines of text that conform to the pattern.

Egrep,fgrep

Pattern: Patterns in English, the text word wildcards regular the expression of the meta-character combination to form a matching condition.

grep [Optians] [pattern] File ...

-I--ignore-case ignoring case

--color

-V Reverse lookup rows that are matched by a pattern are not displayed

-O displays only strings that are matched by the pattern

*: Any character of any length

？ : Any single character

[]: Within the specified range

[^]: outside the specified range

Regular expression: Regular expression,regexp

grep ' pattern ' file

Metacharacters

.: matches any single character;

[]: matches any single character of the specified range

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

[: lower:],[:upper:],[:p unct:],[:space:],[:alpha:],[:alnum:],[:d igit:]

Number of matches:

*: Matches any of its preceding characters any time

A, B, AB, AAB,ACB,ADB,AMNB

A*b

A.*b

. *: Any character of any length

\？ : Make a match, match its preceding character once or 0 times

Normal work in greedy mode, as long as possible to match

\{m,n\}: Matches its first character at least m times, up to N times

\{1,\} at least once

\{0,3\} up to three times

grep ' A\{1,3\}b A

Location anchoring:

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

grep ' ^r. T '/etc/passwd

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

grep ' y$ '/etc/inittab

grep ' B. h$ '/etc/passwd

^$: Blank Line

Anchor Word head:

\<: Any character following it must be a word, the first appears, the anchor word

\>: Any character in front of it must appear as a word, trailing, anchor ending

\<root\>

\broot\b =\<root\>, can use B instead of <,>

Group:

\(\)

\ (ab\) * AB can appear 0 times, 1 times, any time

grep ' \ (L.. e\). *\ (L.. e\) ' A matches with L. E, and the middle is any character

\1: Refers to the first opening parenthesis and all the contents of the corresponding closing parenthesis

[2]

[3]

grep ' \ ([0-9]\). A number appears in the *\1$/etc/inittab line and ends with the same number

grep ' \ (^[0-9]\). *\1$/etc/passwd with first number of switches and ending with the same number

Exercise: Analyzing the characteristics of the first two lines in the following file in the/etc/inittab file (the number must be the same in each row), write a pattern that can be found exactly like two lines:

11:1:WAIT:/ETC/RC.D/RC 1

13:3:WAIT:/ETC/RC.D/RC 3

grep ' ^1\ ([0-9]\). \1.*\1$ '

Regular Expressions:

Basic REGEXP: Basics

Extended REGEXP: Extended

Basic Regular Expressions:

.: Any single character

[]: Specify Range

[^]: outside the specified range

Number of matches:

*: any time before it

\?:0 Times or 1 times

\{m,n\)}: At least m times, to more than n times

. *: Characters of any length

Anchoring:

^: Beginning of the line

$: End of line

\<,\b: Any subsequent word must appear in the header

\>,\b: Any word in front must appear in the tail

\ (\) for grouping

\1,\2,\3 ... for a back reference

grep: A command that filters text using patterns defined by a basic regular expression

-I.

-V Reverse search, mismatched display

-O displays only the matched string

--color

-E: Using extended regular expressions

-a# (numeric): When a row is matched by a pattern specified by grep,

Grep-a 2 ' ^core ID '/proc/cpuinfo followed by 2 lines

-B:

Grep-b 2 ' ^core ID '/proc/cpuinfo Front 2 lines

2 rows before and after the-C:-C2

To extend the regular expression:

Character matching: Same as basic

.

[]

[^]

Number of Matches

*: Same as basic

？ : Its previous or 0 times

+: Its preceding character at least once, matching its preceding character at least 1 times

+* approximate *

{M,n}, no need to use \, use directly

Position anchoring is the same as the basic

Group:

(): Group

\1,\2,\3,....

|: Or: The meaning of a|b or

grep--color-e C|cat A.txt

Egrep ' \b ([0-9]|[ 0-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-9]\b '

grep Regular Expression