Grep Regular Expression

Grep Regular Expression

A regular expression is a symbolic representation used to recognize text patterns. In Linux, the main program for processing regular expressions is grep. Grep searches for the rows matching the Regular Expression and delivers the results to the standard output.

1. grep matching mode

Grep accepts options and parameters as follows (regex indicates a regular expression)

1	grep [options] regex [files]

The options table is as follows:

Option	Description	Function Description
-I	Ignore case	Case Insensitive
-V	Invert match	Unmatched
-L	File-with-match	Output matched file name
-L	File-without-match	Output unmatched file name
-C	Count	Number of matched output rows)
-N	Number	Add the file name and the number of rows in the file name before the output matching row.
-H	No-filename	Suppress output of file names

Example

Assume that there are three files del1, del2, and del3. The content of these files is as follows:

Example

2. special characters

Symbol	Description	Example
^	Start mark	Examples of "^ abc" satisfying abc and abcd
^	Non ()	"[^ Abc]" satisfying examples: ddd, mpd
$	End mark	Example abc and mmabc
.	Any character	Example abc, fapcc
<	Match a word	Examples of "<abc" satisfying abc and abcd
>	Match word ends	Example abc and pmrabc
|	Or	Example AAA, BBBpp

3. Range

Symbol	Description	Example
?	Match the first character 0 or 1 time	"Abc ?" Example AB and mabcd
*	Match the first character ≥ 0	Example of "abc *" satisfying abbb and abcdk
+	Match the first character ≥ 1	Example of "abc +" satisfying abcd and abcccdd
{}	{M}, {m, n}, {m,}, {, n} match the previous character m times, m to n times, ≥m times, ≤n times respectively.	Examples of "abc {3, 5}" fulfillment: abcccc and abcccccc
[]	[] If it is not a range, select one. If it is a range, select one.	"M [abc] p" satisfies the example acpd; m [1-9] p satisfies the example m8pp
()	Put all the elements of the candidate in () and separate them with |	Examples of "a (1 | 2 | 3) bc" satisfying a1bc and mba3bcd

Note: {} needs to be transferred in Zheng's expression, but {} () does not.

Note:

4. Standard character classes

Character class	Meaning
[: Alnum:]	Letters and numbers, equivalent to [A-Za-z0-9]
[: Word:]	[: Alnum _
[: Alpa:]	Letter, equivalent to [A-Za-z]
[: Digit:]	Number, equivalent to [0-9]
[: Xdigit:]	Hexadecimal character, equivalent to [0-9A-Fa-f]
[: Blank:]	Spaces and tabs
[: Graph:]	Visible characters, up to 33 ~ 126
[: Lower:]	Lowercase letters
[: Upper:]	Uppercase letters
[: Print:]	Printable characters
[: Space:]	Blank characters, equivalent to [trnvf]
[: Punct:]	Punctuation Marks
[: Cntrl:]	ASCII control code, including 0 ~ 31, 127

Example

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