Linux--grep and regular Expressions (II.)

There are two types of regular expressions:

Basic REGEXP: Basics

Extended REGEXP: Extended

Basic Regular Expressions:

. : Any single character

[]: A single character in the specified range

[^]: Specify a single character outside the range

Number of matches:

*: The characters in front of it any time

\?:0 or 1 times

\{m,n\}: At least m times, up to N times

. *: Any character of any length

Anchoring:

^: Anchor at the beginning of the line

$: End of line anchoring

\<: Anchor Word first

\>: Anchor ending

\ (\): Grouping

\1,\2,\3,...

grep: A command that filters text using patterns defined by a basic regular expression

-I: Case insensitive

-V: Reverse match

-O: Show only the matching string

--color: Matching to the highlighted display

-E: Using extended regular expressions

-A #: When a row is matched to a pattern specified by grep, the line is displayed, followed by

-B #: Ditto, not only the matching line shows, the front also shows

-C #: Ditto, not only the matching line shows, the upper and lower # lines are also displayed

To extend the regular expression:

Character Matching:

. : Matches a single character

[]: A single character in the specified range

[^]: Specify a single character outside the range

Number of matches:

*; its preceding character any time

? : Its former character 0 or 1 times

+: Match the characters in front of it at least once

{M,n}

Location anchoring:

^

$

\<

\>

Group:

(): Group

\1,\2,\3,...

Or

| : OR

C|car:c or Cat

GREP-E = Egrep

Match white space characters at least once

# grep-e ' ^[[:space:]]+ '/etc/passwd

Find the number between 1-255 in the/boot/grub/grub.conf file;

# egrep ' \< ([1-9] | [1-9] [0-9] | 1 [0-9] [0-9] | 2 [0-4] [0-9] | [0-5]) \> '/boot/grub/grub.conf

IPv4:

Five categories:

A:1-127

b:128-191

c:192-223

D

E

Match IP:

# \< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[01][0-9]|22[0-3]) \> (\.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-4] \>) {2}\.\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-4]) \>

Grep,egrep

Fgrep: Regular expressions are not supported

Linux--grep and regular Expressions (II.)