Regular Expression in Shell

I. Regular Expression description

· Concept: Describe data with common attributes with a string of characters

· Note: when using regular expressions for matching, be sure to use the \ transfer symbol.

· Format:

A. grep [Option] 'regular expression' file list 1 file list 2

B. | pipeline commands

· Processing Method

A. Unit of Action Processing

B. By default, the screen of the row knife that matches the regular expression is output.

C. Perform row-by-row processing on the data. After the current row is processed, the next row is automatically processed until the end of the process.

· Options

→ -- Color highlight matched data

→-Q: The matching result is not displayed.

→-C: count the number of rows that match the matched rows

→-V reverse matching

→-N: displays the row number that matches the matching row.

→- Case-insensitive letters are ignored during I matching.

→-E extension match, or use egrep

→ [] Match within the range. You can use the hyphen "-" to match any character in the range.

Ii. Matching instances

· Matching letters:

→ [Dota] # match D, O, T, or

→ [A-Z] # match any character in A to Z, the same way [A-Z]

→ [A-Z] # match all letters

· Matching numbers

→ [139] # match 1, 3, or 9

→ [0-9] # match all numbers

→ [A-Z0-9] # match all numbers and letters

· Match symbols

→ [?! _-] # Do not write the "-" symbol between other characters. Otherwise, it will be considered as a hyphen without matching.

→ \ <# Match the start of a word

→ \> # Match the end of a word

# Cat text # compile a text by yourself. Here are a few simple examples: Use | pipelines or use grep directly.

Dota

ABCD

ABCD

1234

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

# Cat text | grep Dota

Dota

# Cat text | grep [Dota]

Dota

ABCD

Abcd1234

A1b2c3d4

Thank you for watching

# Grep [A-Z] Text

Dota

ABCD

ABCD

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

# Grep "\ <TH" Text

Thank you for watching

# Cat text | grep "ing \>"

Thank you for watching

# Cat text | grep [-]

?! _-

# Cat text | grep-e 'dota | 808080'

Dota

1234

Abcd1234

# Grep-e 'dota | ^ 1234 $ 'text

Dota

1234

· Matching range

→ ^ [0-9] # match the row starting with a number

→ [^ 0-9] # obtain the reverse matching row, ^ indicates 'reverse select' in'

# Cat text | grep ^ [0-9]

1234

1a2b3c4d

# The matched rows will still match the rows containing numbers because the matched rows contain non-numeric characters.

# Cat text | grep [^ 0-9]

Dota

ABCD

ABCD

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

?! _-

# This is the same as [^ 0-9 ].

# Grep [^ A-Z] Text

ABCD

1234

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

?! _-

# Cat text | grep [^ A-Z]

Dota

ABCD

1234

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

?! _-

# Cat text | grep [^ A-Z]

1234

Abcd1234

A1b2c3d4

1a2b3c4d

Thank you for watching

?! _-

3. metacharacters

· Metacharacter (Regular Expression): a regular expression is composed of one or more metacharacters.

· Metacharacter classification: character matching, frequency matching, location anchoring, grouping, etc.

→^# First line Matching

→$# Line-end matching

→^ $# Empty row match

→.# Match any single character (except line break \ n)

. $ # Match rows ending with any character

^. $ # Match the row ending with any character

^ X. y $ # match rows containing x. y. Here. represents any character

\. $ # Match rows ending with a character.

# Cat text | grep '^ do'

Dota

# Grep '34 $ 'text

1234

Abcd1234

# Cat text | grep 'W. t'

Thank you for watching

· Set the matching times and format of Regular Expressions

→ * # Match the regular expression that appears zero or multiple times

→? # Match the previous regular expression, zero or one occurrence

→ + # Match the regular expression that appears once or multiple times

→ () # Match the expression as a whole

→ {N, m} # pointing to the regular expression above, the range of matching times

{N ,}# match times greater than or equal to n

{, M} # match times less than or equal to m

{X} # match times equal to X

# Cat repetition

ABC

Abcabc

Abcabcabc

# Grep "\ (ABC \) \ {1 \}" repetition

ABC

Abcabc

Abcabcabc

# Grep "\ (ABC \) \ {3 \}" repetition

Abcabcabc

# Grep "\ (ABC \) \ {2, 3 \}" repetition

Abcabc

Abcabcabc

# Grep "\ (ABC \) \ {2, \}" repetition

Abcabc

Abcabcabc

Some practical examples

· Matching email addresses

# Grep "[a-Z0-9 _] \ {\} @ [a-Z0-9] \ {\} \ (\. com \) \ | \ (\. cn \)" 1.txt

[Email protected]

· Matching IP addresses

# Grep "\ ([0-9] \ {1, 3 \}\. \) \ {3 \} [0-9] \ {1, 3 \} "/etc/sysconfig/network-scripts/ifcfg-eth0

Ipaddr = 172.16.8.8

Netmask = 255.255.255.0

Gateway = 172.16.8.1

Dns1 = 192.168.8.1

· Match the MAC address

# Grep "\ ([0-9a-fa-f] \ {2 \ }:\) \ {5 \} [0-9a-fa-f] \ {2 \}"/etc/sysconfig/network-scripts/ifcfg-eth0

Hwaddr = 00: 0C: 29: 40: 74: D7

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