Linux Regular expressions

A regular expression is a notation notation used to recognize text patterns. The main program for Linux to process regular expressions is grep. grep searches for rows that match the regular expression and transports the results to standard output.

1. grep matching mode

grep accepts options and parameters as follows (where Regex represents a regular expression)

grep [Options] regex [Files]

The options are the following table:

Options	Meaning	Function description
-I.	Ignore case	Ignore case
-V	Invert match	Does not match the matching
-L	File-with-match	The output matches the file name
-L	File-without-match	Output mismatched file names
-C	Count	Number of output matches (number of rows)
-N	Number	The output matches the row with the file name and the number of rows in the file name
-H	No-filename	Suppress the output of a file name

Examples Show

Suppose there are three files Del1, Del2, del3 three files with the contents as follows

Example

2. Special characters

Symbol	Meaning	Example
^	Start tag	Examples of "^ABC" satisfies ABC, ABCD
^	Non (within [])	Examples of "[^ABC]" gratification: DDD, MPD
$	End tag	Examples of "abc$" satisfies ABC, MMABC
.	Any character	Examples of "A.C" satisfies ABC, FAPCC
\<	Match Word start	Examples of "\<ABC" satisfies ABC, ABCD
\>	Match Word End	Examples of "abc\>" satisfies ABC, PMRABC
|	Or	"Aaa| BBB "satisfies the example AAA, BBBPP

3. Scope

Symbol	Meaning	Example
?	Match the previous character 0 or 1 times	"ABC?" Examples of satisfaction AB, MABCD
*	Match previous character ≥ 0 times	Examples of "abc*" gratification abbb, ABCDK
+	Match previous character ≥ 1 times	Examples of "abc+" satisfies ABCD, 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\}" gratification ABCCCC, ABCCCCCC
[]	[] If not within the scope, select one;	"M[abc]p" satisfies the example acpd;m[1-9]p satisfies the example m8pp
()	Place all candidate elements within (), separated by |	"A (1|2|3) BC" Satisfied Example A1BC, MBA3BCD

Note: {} needs to be transferred in the Zheng expression, and {} () is not required.

Note An example of the scope of the {}:

4. Standard character class

Character class	Interpretation
[: Alnum:]	Letters and numbers, equivalent to [a-za-z0-9]
[: Word:]	[: Alnum:] Plus underline _
[: ALPA:]	Letters, equivalent to [a-za-z]
[:d Igit:]	Number, with [0-9] equivalent
[: Xdigit:]	hexadecimal characters, equivalent to [0-9a-fa-f]
[: Blank:]	Spaces and Tabs
[: Graph:]	Visible characters, by extension 33~126
[: Lower:]	lowercase letters
[: Upper:]	Capital
[:p rint:]	printable characters
[: Space:]	White space character, equivalent to [\t\r\n\v\f]
[:p UNCT:]	Punctuation
[: Cntrl:]	ASCII control code, including character 0~31 and 127

Example

Linux Regular Expressions