Grep, regular expression, and grep Regular Expression

Grep, regular expression, and grep Regular Expression

Grep: a text search tool that checks the row-by-row matching of the target file based on the user-specified "Mode": prints the matched rows.

Pattern: filtering condition for writing regular expressions.

Regular Expression (REGEXP): A pattern written by a special character or text character. Some of these characters do not represent the literal meaning of the character, but represent the control or wildcard function.

There are two types:

Basic Regular Expression: BRE

Extended Regular Expression: ERE (grep-E, egrep)

Use of grep:

Grep [Option] mode [file...]

Option:

-- Color = auto: color the matched text;

-V: displays the rows that are not matched by the "mode;

-I: case-insensitive characters are ignored;

-O: only the matched strings are displayed;

-Q: In silent mode, no information is output;

-A #: display matching rows and backend # Rows;

-B #: displays matching rows and the first # rows;

-C #: displays matching rows and front and back # rows;

-E: Use ERE (equivalent to egrep)

 

Basic Regular Expression metacharacters:

Character match:

.: Match any single character;

[]: Match any single character in the specified range

[^]: Match any single character out of the specified range

[: Alnum:]: represents English case characters and numbers, and a-z, A-Z, 1-9

[: Alpha:]: stands for English case characters, and a-z, A-Z

[: Cntrl:]: indicates the control buttons, such as CR, LF, Tab, and Del.

[: Digit:]: represents a number, 1-9

[: Graph:]: All keys except space and Tab

[: Lower:]: lowercase letter, a-z

[: Print:]: represents any character that can be printed.

[: Punct:]: stands for punctuation marks, that is "'?!; : # $

[: Space:]: any character that produces white space, including space key [Tab] CR

[: Upper:]: stands for capital letters, A-Z

[: Xdigit:]: represents the hexadecimal number type, so contains 0-9, A-F, a-f digits and characters

Note: The preceding [: ***:] must be used in double brackets, that is, [[[: ***:]

 

Matching times:

*: Matches the previous character 0 or multiple times;

.: Match any single character;

. *: Matches any character 0 or multiple times;

\? : Match the first character 0 or 1 time;

\ +: Match the previous character once or multiple times;

\ {M \}: match the previous characters m times;

\ {M, n \}: match the previous character at least m times, at most n times;

\ {0, n}: match the preceding characters up to n times;

\ {M ,\}: match the previous character at least m times;

 

Positioning:

^: The first line is anchored to the left of the mode;

$: The end of the line is anchored to the rightmost of the mode;

\ <Or \ B: the beginning of the word, used on the left side of the word mode;

\> Or \ B: The end of the word, used to the right of the word mode

 

Group and Back Reference:

\ (\): Binds one or more characters for processing as a whole.

Backward reference: Apply the pattern matching characters (not the pattern itself), \ 1, \ 2... \ n

For example, \ (abc * \ (def \) ghe \? \)

\ 1: abc * \ (def \) ghe \? Corresponding to the content in the brackets where the first left parenthesis is located

\ 2: def corresponds to the content in the brackets of the second left bracket

Extended Regular Expression metacharacters:

Same as the regular expression: the difference is that special symbols do not need to be escaped using .? Equal \?, () Equals \ (\), {m} equals \ {m \}, and + equals \ +. Note: \ <, \>, \ B still needs to add "\"

N | m: | match all content on the left or right

For example:

Abc | def matches abc or def. By default, all characters on the left and right sides are matched and can be grouped by (), as shown below:

AB (c | de) f matches c | de