Regular expression grep egrep

Regular expression: Regual expression,regexp

A pattern written by a class of special characters and text characters, where some characters do not represent their literal meanings, but are used to denote control or the function of a wildcard.

Divided into two categories:

Basic Regular Expressions: BRE

Extended Regular expression: ERE

Grep:global search REgular expression and print out of the line

Function: The text Search tool, according to the user-specified "mode (filter)" to match the target text line by row to check, print the matching line.

Pattern: The filter condition written by the metacharacters and text characters of the regular expression.

Regular expression engine:

grep [options] pattern [file]

grep [Options]-E pattern [-f file] [file ...]

-I: Ignore character case

-O: Show only the string that matches to itself

-V: Shows rows that cannot be matched to

-E: Support for using extended regular expressions

-W: The whole line matches a word (one word can be repeated on a line)

-Q: Silent mode, do not output any messages

-R: Recursively find each directory

-C: Count rows

-E: Specifies a string as a template style for finding the contents of a file

-A #: After after # lines

-B #: before Ago # line

-C #: # lines before and after the context

-N: Match to line plus line number

Basic regular Expression meta-characters:

Character Matching:

. : Any single character

[]: matches any single character of the specified range

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

Number of matches:

Used after the character that you want to specify the number of occurrences. Used to limit the number of occurrences of the preceding character, which works by default in greedy mode.

*: matches the preceding character any number of times: 0, 1, multiple times

. *: Matches any character of any length

\? : matches the preceding character 0 or 1 times, that is, the preceding character is optional

\+: Matches its preceding character 1 or more times

\{m\}: Matches the preceding character m times

\{m,n\}: Matches the preceding character at least m times, up to N times

\{0,n\}: Up to n times less than or equal to n

\{m,\}: At least m times greater than or equal to M

Location anchoring:

^: Anchor at the beginning of the line for the leftmost mode

$: End of line anchoring

^...$: Match whole line

^$: Blank line ^[[:space:]]$ lines that contain white space characters

\< or \b: The first anchor of the word

\> or \b: Final anchoring

\<pattern\>: Anchoring complete words

Grouping and referencing:

\ (\) Escapes multibyte parentheses, bundling multiple characters together as a whole

The contents of the pattern in the grouping brackets are automatically recorded in the internal variables by the regular expression engine, which are:

\1: First set of parentheses (first parenthesis from left)

\2: Second set of parentheses

Cases:

grep "\ (L.. e\). *\1 "Love.txt

Egrep

Support for extended regular expression implementations similar to grep text filtering,-G supports general regular expressions

Egrep [OPTIONS] PATTERN [FILE ...]

Extend the metacharacters of regular expressions:

. : Any single character

[]: matches any single character of the specified range

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

Number of matches:

Used after the character that you want to specify the number of occurrences. Used to limit the number of occurrences of the preceding character, which works by default in greedy mode.

*: matches the preceding character any number of times: 0, 1, multiple times

? : matches the preceding character 0 or 1 times, that is, the preceding character is optional

+: Match its preceding character 1 or more times

{m}: matches the preceding character m times

{M,n}: matches the preceding character at least m times, up to N times

{0,n}: up to n times less than or equal to n

{m,}: At least m times greater than or equal to M

Location anchoring:

^: Anchor at the beginning of the line for the leftmost mode

$: End of line anchoring

\< or \b: The first anchor of the word

\> or \b: Final anchoring

Grouping and referencing, with grep

Or:

A|b cat| C means the entire left or the right side.

Practice:

Find all the lines in the/proc/meminfo file that begin with uppercase or lowercase s, at least three ways:

Grep-i "^s"/proc/meminfo

grep "^[ss]"/proc/meminfo

Grep-e "^ (s| S) "/proc/meminfo

Displays information about Roo, CentOS, or user users on the current system

Grep-e "^ (root/centos/user) \>"/etc/passwd

Find the line at the beginning of the/etc/rc.d/init.d/functions file that follows a word (including an underscore) followed by a parenthesis

GREP-E-O "[_[:alnum:]]+\ (\)"/etc/rc.d/init.d/functions

Use the echo command to output an absolute path, using Egrep to remove the base name

echo/etc/sysconfig/| GREP-E-O "[^/]+/?$"

Find the value between 1-255 in the ifconfig command result

Ifconfig | GREP-E-O "\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \> "

Find all IPV4 addresses in the ifconfig command result

Ifconfig | GREP-E-O "(\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>\.) (\< ([0-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>\.) {2}\< ([0-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \> "

Find the row for the user name and shell name in the/etc/passwd file

Grep-e "^ ([^:]+\>). *\1$"/etc/passwd

Find the IP address of the ifconfig eth0

Ifconfig eth0 | grep "inet addr" | Cut-d:-F 2|cut-d ""-F 1

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Regular expression grep egrep