grep Regular Expression

Text Processing Tools

grep function: The text Search tool, according to the user-specified "mode" to match the target text line by row to check; print matching lines

REGEXP: A pattern written by a class of special characters and text characters, in which some characters (metacharacters) do not represent literal meanings, but are functions that represent control or a wildcard

Parameters:

--color=auto: Coloring the text to match to a display

-V: Displays rows that are not matched by pattern

-I: Ignore character case

-N: Show matching line numbers

-C: Count the number of matching rows

-O: show only the matching string

-Q: silent mode, does not output any information

-A #: after, followed by # lines

-B #: Before, front # line

-C #:context, front and back # lines

-E: Implementing a logical or relationship between multiple options # #grep –e ' cat '-e ' dog ' file

-W: Entire line matches whole word

-E: Use ERE

-F: equivalent to Fgrep, does not support regular expressions

Character Matching :

. match any single character;

[] matches any single character within the specified range

[^] matches any single character outside the specified range

brackets are added to the front, Egrep don't need

[: Alnum:] Letters and Numbers

[: Alpha:] represents any English uppercase and lowercase characters, i.e. A-z,a-z

[: Lower:] Lowercase Letters

[: Upper:] Uppercase Letters

[: Blank:] white space characters (spaces and tabs)

[: Space:] horizontal and vertical whitespace characters (compared to [: Blank:] wide range of inclusions)

[: Cntrl:] non-printable control characters (backspace, delete, alarm ...) )

[:d Igit:] decimal Digits [: Xdigit:] hexadecimal digits

[: Graph:] non-whitespace characters that can be printed

[:p rint:] printable characters

[:p UNCT:] Punctuation

[: Alnum:]: letters and Numbers

Number of matches: used after the number of characters to be specified, to specify the number of occurrences of the preceding character

* match the preceding character any time, including 0 times

Greedy mode: Match as long as possible

. * Any character of any length

\? match its preceding character 0 or 1 times

\+ matches the characters in front of it at least 1 times

\{n\} matches the preceding character n times

\{m,n\} matches the preceding character at least m times, up to n times

\{,n\} matches the preceding character up to n times

\{n,\} matches the preceding character at least n times

Group

Group:\ (\) binds one or more characters together and treats them as a whole, such as:\ (root\) \+

The contents of the pattern in the grouping brackets are recorded in internal variables by the regular expression engine , which are named: \1, \2, \3, ...

\1 represents the character that matches the pattern between the first opening parenthesis and the matching closing parenthesis from the left

Example:\ (string1\+\ (string2\) *\)

\1 :string1\+\ (string2\) *

\2 :string2

Position anchoring: positioning where it appears

^ beginning of the line anchor, for the leftmost mode

$ Line end anchor for the right side of the pattern

^pattern$ for pattern matching entire row

^$ Empty Line

^[[:space:]]*$ Blank Line

\< or \b The first anchor for the left side of the word pattern

\> or \b ending anchor; for the right side of the word pattern

\<pattern\> Match Whole word

Egrep Extended Regular Expressions

inherit all pattern matches of grep, just no longer need \ as escape character

Egrep One more or (to use greater than sign):

A|b:a or b

C|cat:c or cat

(c|c) At:cat or Cat

Practice:

1. Display the lines in the/proc/meminfo file that begin with a non-differentiated s;

Grep-i "^s"/proc/meminfo

Egrep "^ (s|s)"/proc/meminfo

2. Display the line ending with Nologin in/etc/passwd;

grep "/sbin/nologin$"/etc/passwd

3, display/etc/inittab in the beginning of #, and followed by one or more white space characters, followed by any character line;

grep "^#[[:blank:]]\{1,\}.*"/etc/inittab

4, display/etc/inittab contains: A number: (that is, two colons in the middle of a number) line;

grep--color "\:[[:d igit:]]\:"/etc/inittab

5. Display lines that start with one or more whitespace characters in the/boot/grub/grub.conf file;

grep "^[[:blank:]]"/boot/grub/grub.conf

6. Display a line in the/etc/inittab file that begins with a number and ends with a number that is the same as the beginning number;

grep "^\ ([[:d igit:]]\). *\1$"/etc/inittab

7, find out a file, 1 digits, or 2-digit number;

Egrep "\<[[:d igit:]]{1,2}\>"/etc/passwd--color

8. Display three user root, Bin, ftp uid and default shell

Egrep "^ (root|bin|ftp)"/etc/passwd |cut-d:-f1,3,7

9. Find the beginning of a word in the/etc/rc.d/init.d/functions file

grep "^\<[[:alpha:]]"/etc/init.d/functions

10. Display all IPV4 addresses in ifconfig command results

Ifconfig |egrep-o "([[:d igit:]]{1,3}.) {3} [[:d Igit:]] {1,3} "

11. Remove the base name of the/etc/init.d/network

Ls/etc/init.d/network |egrep-o "[^/]+$

12, take out the path of/etc/init.d/network

Ls/etc/init.d/network |egrep-o ". * *"

grep Regular Expression