Regular Expressions and grep

Part three regular expressions and grep

This chapter mainly through some application examples, to explain the regular expression.

1 Regular Expressions

A regular expression is an expression of a string. It can represent one or more columns of strings with meaningful special symbols.
grep is a regular expression tool commonly used under Linux systems, and you can use grep to retrieve strings for input streams such as text.

2 Regular Expression special symbols

Refer to the table below

3 grep Expressions

Basic Format
grep [OPTIONS] PATTERN [FILE ...]
Format description
Pattern: Match pattern. Can be a string, or it can be a regular expression.
[FILE ...] : Is the file (set) of grep search
[Options]: is the option for grep. Common options have the following options.
-C: Outputs only the count of matching rows.
-I: Case insensitive (only for single-character).
-H: The file name is not displayed when querying multiple files.
-L: Only file names that contain matching characters are output when querying multiple files.
-N: Displays matching lines and line numbers.
-S: does not display error messages that do not exist or have no matching text.
-V: Displays all lines that do not contain matching text.
- R: When file contains a folder name, all subdirectories of that folder are traversed, and subdirectories are not traversed by default.

4 Application Examples

The following is an example of Input.txt, which describes grep. The text content of Input.txt is as follows:

"Open Source"is a good mechanism to develop programs.apple are my favorite food. Football game isn't use feet only.this dress doesn‘T fit me. However, this dress was about $3183 dollars.^Mgnu isFree Air notFree beer.^Mher hair is very beauty.^MI can‘T finish the test.^moh! The Soup taste good.^Mmotorcycle is cheap than car. This window isClear. The symbol ' *  is represented as start. Oh! my god! the GD software is a library for Drafting Programs.^myou was the best was mean you were the No. 1 "glad ". I like Dog.google are the best tools for search Keyword.goooooogle yes! go! go! Let ' s go       

(01), find the line containing "the", and display the line number.

"the" Input.txt 

Description:-n means the line number is displayed

(02), case-insensitive, find rows that include "the", and display line numbers.

grep-"the" Input.txt  

Description:-n means the line number is displayed;-I means case-insensitive, that is, ignore case.

(03), to find rows that do not include "the", count the number of rows.

"the" Input.txt 

Description:-C Represents a statistic (count), and-V indicates an item that does not match.

(04), find the file containing "the" in "Current directory" and "All subdirectories", and display the line number of "the" in it.

"the". 

Description:-r means recursive lookup;-n indicates the line number is displayed.

(05), find the item that matches "T?st", where? is any character.

"t.st" Input.txt 

Description:. means match any character

(06) To find rows that contain numbers

"[0-9]""[[:d igit:]]     

Description: [0-9] Represents a number between 0-9 and [[:d Igit:]] Also represents a number between 0-9

(07), find the line that begins with the

"^the" Input.txt 

Description: "^the" means starting with the

(08), find the line that ends with a lowercase letter.

"[a-z]$" Input.txt 

Description: [A-z] represents a lowercase letter, $ represents a terminator, and [a-z]$ represents an item that ends in a lowercase letter.

(09) To find a blank line.

"^$" Input.txt 

Description: ^ denotes the beginning, such as ^t, which begins with the letter T; $ represents the end, as e$ means ending with E. ^$ represents a blank line.

(10) To find words that begin with the letter G

"\<g" Input.txt 

Description:\< represents the beginning of a word, and \<g represents a word starting with G.

(11), find the word "go" for the string. Note: Strings such as Goo,good cannot be included

"\<go\>" Input.txt 

Description:\< Indicates the beginning of a word,\> means the end of the word. \<go\> denotes the letter G, ending with the letter O.

(12), find a line that includes 2-5 letters O.

"o\{2,5\}" Input.txt 

Description: Pattern\{n,m\} represents n to M pattern. O\{2,5\} represents 2-5 letters O.

(13), find rows that include more than 2 letters O (including 2).

"ooo*""oo\+""o\{2,\}" Input.txt       

Description
Ooo*: The front two oo means match 2 letter O, and the following o* indicates matching 0 to multiple letters O.
oo\+: The first letter o means matching a single letter o; the last "o\+" works together, where the \+ is an escaped +, representing 1 to more, while o\+ represents 1 to more letters O.
Pattern\{n,\} indicates more than n pattern. O\{2,\} means more than 2 letters O.

4 Egrep4.1 egrep Description

Egrep is an extended grep, meaning it functions more than grep. "Egrep" is equivalent to "GREP-E".
Egrep with grep, supports parentheses "()" And operator "|" (expressed or).

4.2 Egrep Application Example

Still use the above input.txt as a description of the input text

(01), find the line containing the or this

"the|this" Input.txt 

Description:-n Indicates the line number of the output match, and "the|this" means the item that includes the.

(02), find the line containing the or this

"(the|this)" Input.txt 

Description:-N Represents the line number of the output match, and "the|this" means the item that includes the or this, and-V represents the opposite of the match. That is,-V "the|this" means that neither the item nor the this is included.

Regular Expressions and grep