Three minutes to learn about regular expressions in the grep command

Three minutes to learn about regular expressions in the grep command

In Linux and Unix-like operating systems, how does one use the grep command with a regular expression?

Linux provides the GNU grep tool that supports extended regular expressions. By default, GNU grep is installed in all Linux systems. The grep command is used to retrieve text information from any location on a server or workstation.

I. Quick introduction to Regular Expressions

1. How to match the content you want to search?

A regular expression is only a pattern that matches each input line. Mode is a character sequence. The following are examples:

For example, "^ w1", "w1 | w2", and "[^]".

Search for 'vivek' in '/etc/passswd '.
Grep vivek/etc/passwd

Output result case:
Vivek: x: 1000: 1000: Vivek Gite,:/home/vivek:/bin/bash
Vivekgite: x: 1001: 1001:/home/vivekgite:/bin/sh
Gitevivek: x: 1002: 1002:/home/gitevivek:/bin/sh

Search for 'vivek' in any situation (that is, no size difference ):
Grep-I-w vivek/etc/passwd

Retrieve 'vivek' and 'l1' case-insensitive ':
Grep-E-I-w 'vivek | r1'/etc/passwd

In the last example, the extended regular expression mode is used.

Fixed search content location:
You can use the ^ and $ symbols to force a regular expression to match the start or end position of a row. The following example shows the text starting with 'vivek.
Grep ^ vivek/etc/passwd

Output result example:
Vivek: x: 1000: 1000: Vivek Gite,:/home/vivek:/bin/bash
Vivekgite: x: 1001: 1001:/home/vivekgite:/bin/sh

You can only display text lines starting with vivek. For example, it means that the start of a word such as vivekgite and vivekg is not displayed.
Grep-w ^ vivek/etc/passwd

Retrieve the text format ending with 'foo:
Grep 'foo $ 'FILENAME

You can also use the following method to search for blank rows:
Grep '^ $' FILENAME

2. How to match specific characters?

Match 'vivek' or 'vivek ':
Grep '[vV] ivek' FILENAME

Alternatively, you can:
Grep '[vV] [iI] [Vv] [Ee] [kK] 'filename

You can match a number (for example, vivek1 or Vivek2 ):
Grep-W' [vV] ivek [0-9] 'filename

You can match two digits (for example, foo11 and foo12 ):
Grep 'foo [0-9] [0-9] 'filename'

It's not just a number. You can match letters:
Grep '[A-Za-z]' FILENAME

Display All text lines containing letters w or n:
Grep [wn] FILENAME

In the expressions in parentheses, the name of the character class included in "[:" and ":]" indicates a list Of all characters belonging to the class. Standard character class name:
[: Alnum:]-alphanumeric characters.
[: Alpha:]-alphabetic order
[: Blank:]-space and tab.
[: Digit:]-number: '0 1 2 3 4 5 6 7 8 9 '.
[: Lower:]-lowercase letter: 'a B c d e F '.
[: Space:]-special characters: tab, line break, vertical tab, form feed, carriage return, and space.
[: Upper:]-uppercase letter: 'a B c d e f g h I J K L M N O P Q R S T U V W X Y Z '.

In the following example, all uppercase letters are matched:
Grep '[: upper:]' FILENAME

3. How do I use wildcards?

You can use "." To replace a single character. In the following example, all three words starting with the letter "B" and ending with the letter "t" are queried.
Grep '\ <B. t \> 'filename'

In the preceding example,
\ <Match a space string at the beginning of a word
\> Match a space string at the end of a word

Search and output the results of all two letters:
Grep '^ .. $' FILENAME

Search and display all results starting with '.' And numbers:
Grep '^ \. [0-9]' FILENAME

Escape Character '.'
The following regular expression finds the IP address 192.168.1.254 and cannot obtain the expected results:
Grep '1970. 168.1.254 '/etc/hosts

All three points must be escaped:
Grep '2017 \. 192 \. 1 \. 100'/etc/hosts

The following example matches only one address:
Egrep '[[: digit:] {1, 3 }\. [[: digit:] {1, 3 }\. [[: digit:] {1, 3 }\. [[: digit:] {1, 3} 'filename

The words Linux or Unix are matched in case-insensitive mode:
Egrep-I '^ (linux | unix)' FILENAME

2. Explore grep advanced search mode in Depth

1. How to retrieve a pattern that starts?

Use the-e Option to search for all results matching '-test. Grep will try to parse '-test-' as an option:
Grep-e '-- test -- 'filename

2. How to Use the OR logical operation in grep?
Grep-E 'word1 | word2 'FILENAME
### OR ###
Egrep 'word1 | word2 'FILENAME

Or you can do this.
Grep 'word1 \ | word2 'FILENAME

3. How to Use the AND logical operation in grep?

Follow the following syntax to display all results that contain the words 'word1 'and 'word2:
Grep 'word1 'filename | grep 'word2'

Alternatively, you can:
Grep 'foo. * bar \ | word3. * word4' FILENAME

4. How to test the sequence?

You can use the following syntax to test the number of repetitions of a character in the sequence:
{N}
{N ,}
{Min, max}

Match the string containing two letters v:
Egrep "v {2}" FILENAME

In the following example, the search file contains the string results of "col" and "cool:
Egrep 'co {1, 2} l' FILENAME

In the following example, match results with at least three letters c:
Egrep 'C {3,} 'FILENAME

The following example matches the phone number in the format of "91-1234567890" (that is, "two digits-ten digits ")
Grep "[[: digit:] \ {2 \} [-] \? [[: Digit:] \ {10 \} "FILENAME

5. How to highlight grep output results?

Use the syntax of the following example:
Grep -- color regex FILENAME

6. How can I make the grep output show only the matched part instead of the whole line?

Use the syntax of the following example:
Grep-o regex FILENAME

Iii. Regular Expression operator Summary

Regular Expression Operator	Description
.	Match any single character.
?	Match the first character 0 or 1 time.
*	Match the first character ≥ 0.
+	Match the first character ≥1.
{N}	Match the first character N times.
{N ,}	Match the first character ≥ m times.
{N, M}	Match the first character N to M times.
-	If the end point of a list or a range is in the list, it indicates the range.
^	Start tag, indicating that an empty string is matched at the start position. It also indicates characters that are not in the list range.
$	End tag. Matches an empty string.
\ B	Word lock. Matches an empty string at the edge of a word.
\ B	Matches an empty string at a non-edge position of a word.
\ <	Matches an empty string starting with a word.
\>	Matches an empty string at the end of a word.

4. grep and egrep

Egrep is grep-E, which interprets the pattern as an extended regular expression. The grep help document defines this as follows:

 In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{,       \|, \(, and \).       Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid  {  in       grep -E patterns and should use [{] to match a literal {.       GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.       For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax  error  in  the  regular  expression.       POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

References:

• Grep and regex help document
• Grep info help document

Grep uses concise and Regular Expressions

Regular Expression usage

Assertion with Zero Width of a regular expression

Linux Command-grep for file text operations

Grep Regular Expression

Regular Expressions and file formatting commands in Linux (awk/grep/sed)

Link: http://www.cyberciti.biz/faq/grep-regular-expressions/
Link: http://www.linuxstory.org/grep-regular-expressions/

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