Regular expression grep egrep fgrep in Linux

Regular Expressions: REGEXP Reguler expression, which uses metacharacters and other strings to describe and match a series of strings that conform to a certain syntactic rule . The default is to match as long as possible (greedy mode).

Metacharacters

.: Matches any single character (in file name wildcard?). Represents any single character)

[]: matches a single character within a specified range

[^]: matches a single character outside the specified range

[: Space:]: white space character

[:p Untt:]: All punctuation

[: Upper:]: All uppercase letters

[: Lower:]: All lowercase letters

[: Alpha:]: All uppercase and lowercase letters

[:d Igit:]: All numbers

[: Alnum:]: All numbers and uppercase and lowercase letters

Character Count matches:

*: matches any number of characters in the preceding character (the file name in the wildcard * denotes any character of any length)

. *: Any character of any length

\?: matches its first character once or 0 times, can be used without

\{\}:\{m,n\} matches its first character at least m times, up to N times

Eg:\{1,\} at least 1 times; \{0,3\} up to 3 times

Location anchoring:

^: Anchor the beginning of the line, after which the character must appear at the beginning

$: Anchor Line end, whose preceding character must appear at the end of the row

^$: Blank Line

\<: The following character must appear as the word header and can be replaced with \b

\>: The first character must appear as the tail of the word, and can be replaced with \b

eg:\<root\>==\broot\b

Group: \ (\)

Eg:\ (a,b\) *:ab appears any time

The main function of the parentheses is not to make the contents of the content be regarded as a whole, but to facilitate the subsequent content reference (back reference)

Eg:\1: Call the first opening parenthesis and its corresponding closing parenthesis to include the contents

\ n: Invokes the preceding nth opening parenthesis and its corresponding closing parenthesis for what is included

Regular expressions: Divided into basic regular expressions and extended regular expressions

To extend the regular expression:

Character Matching:

., [],[^] is the same as the basic regular expression

Number of Matches

*，？ : The same as the basic regular expression? Before you need to add \

+: Its pre-character at least once = = in basic regular expression \ (1,\)

{M,n}: At least m times up to N times, less than the base regular expression curly braces

Anchor character:

\< \> ^ $ \b As with basic regular expressions

Group:

(): No need to add \ support \1,\2,\3,...

Or:

|:or or

Eg:a|b A or B

Eg:c|cat==c or cat (not cat or cat)

grep: Searches for text according to the pattern and displays lines of text that conform to the pattern. a command that uses a basic regular expression and its defined pattern to filter text.

Pattern: The matching criteria for the combination of meta-characters of the expression wildcards regular the patterns text character

-I: Ignore case

--color: Matched to the character plus color display

-O: displays only the string that is matched to

-V: reverse lookup, displaying rows of content that are not matched by a pattern

-E: Using extended regular expressions

-A N: Displays the line that is matched to and the next n rows

-B N: displays the line to which it was matched and its first n rows

-C N: displays the line to which it is matched and the N rows before and after it

Egrep: a command that uses extended regular expressions and its defined patterns to filter text.

Egrep=grep-e

Fgrep:fast grep Quick Find characters do not support regular expressions and are searched by string.

Regular expression grep egrep fgrep in Linux