Regular Expressions and extended Regular Expressions

1. What is a regular expression and what is its function.

Global Search Regular Expression and print out the line.

Regular Expression: Regular Expression Engine

Regular Expression:
Is a type of character writing mode, many of which do not represent its literal meaning, but express control or wildcard functions,

The text search tool performs a row-by-row search on the target file based on the text mode (Search Condition) specified by the user to display matching rows.

Basic Regular Expression: grep

Extended Regular Expressions: egrep, grep-e
Fgrep: fast. Regular Expressions are not supported.

Syntax format:
Grep [Option]... 'pattern' file...

-- Color = auto

Command Options:
-V: Reverse Selection
-O: only display the matched string, not the row of the string.
-I: ignore-Case, Case Insensitive
-E: supports the use of extended regular expressions.
-A # After Matching content # Row Information
-B # Before matching content # Row information (the first matching item is the first line)
-C # Matching content before and after # Row Information

2. Differences between basic regular expressions and extended character expressions

Metacharacter of the basic regular expression: character matching

Times matching metacharacters: used to specify the number of times a character before it can appear

A string of consecutive characters that do not contain special characters is called a word.

Regular number matching metacharacters	Han Yi	Extended regular number matching metacharacters	Han Yi
*	Any length. The first character can appear at any time.	*	Any length. The first character can appear at any time.
\?	0 or 1 time	?	0 or 1 time
\ {M \}	MB	{M}	MB
\ {M, N \}	At least m times, up to n times	{M, n}	At least m times, up to n times
\ {M ,\}	At least m times	{M, 0}	At least m times
\ {0, n \}	Up to n times	{0, n}	Up to n times
.*	Any character of any length	+	At least 1 time

3. Character matching

Character matching	Han Yi
.	Match any single character
[]	Match any single character in the specified range
[[: Alpha:]	Uppercase/lowercase letters
[[: Alnum:]	Uppercase/lowercase letters
[[: Digit:]	Number
[^]	Invert

4. Positioning

Character	Han Yi
^	The first line is anchored at the right of the write mode
$	At the end of the line, write the string at the right of the pattern.
^ $	Blank line
\ <	The beginning of the word, on the left side of the word, \ B \ <char
\>	At the end of the word, on the left side of the word, \ B \ <char

5. Grouping

The pattern matching content in the group can be stored in the memory by the Regular Expression Engine and can be referenced later.

\(\)
Example: \ (AB \)*

Reference:
For example, \ (AB \ (x \) Y \). * \ (Mn \)
Numbered: Left brackets (from left to bottom) and matching right brackets
\ (A \ (B \ (C \) Mn \ (x \). * \ 1

For example:
\ (AB \? C \). * \ 1

Abcmnaaa
Abcmnabc
Abcmnac
Acxyac

Fgrep 'pattern' file...
Regular expressions are not supported.