The use of common regexp in Linux environment

Regular expressions

Shorthand for REGular EXPression
Metacharacters

Number of Matches

Position anchoring

Group

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Metacharacters
. Match any single character

[] matches any single character in the specified range

[^] matches an outside of the specified range
[: Space:] white space characters
[:p UNCT:] punctuation character
[: Lower:] lowercase letters
[: Upper:] Uppercase
[: Alpha:] uppercase and lowercase letters
[:d Igit:] Number
[: Alnum:] numbers and uppercase and lowercase letters

Number of matches (greedy mode)

* Match any of its preceding characters any time
A*b B Front A appears any number of times
A.*b A begins with B ends
. * Any character of any length

\？ Match its preceding character 1 or 0 times meaning that this character is dispensable
A?b
\{m,n\} matches characters preceding it at least m times up to N times
\{1,\} at least once
\{,3\} up to 3 times
A.\{1,3\}b A and B have at least one of three characters

Location anchoring :

^ Anchor Line Any content after this character must appear at the beginning of the line

$ anchor Line end any content that precedes this character must appear at the end of the row
grep ' B. h$ '/etc/passwd
^$ Blank Line
\< any character that follows it must appear as a word header
\> any character in front of it must appear as the tail of the word \b Another way of writing this symbol

Group
\(\)
\ (ab\) * AB can appear any time
\1 the first opening parenthesis and the corresponding group contents

grep ' \ (L.. e\). *\1 ' Test3.txt

Exercise: Analyze the characteristics of the first two lines in the following text in the/etc/inittab file (the number must be the same in each row), write a pattern that can be found exactly like two lines
11:1:WAIT:/ETC/RC.D/RC 1

13:1:WAIT:/ETC/RC.D/RC 3

grep ' ^1\ ([0-9]\): \1.*\1$ '/etc/inittab

grep commands to filter text using patterns defined by the basic regular expression

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Extending regular Expressions

Character matching
. [] [^]

Number of Matches
*
？ No backslashes.
+ match the characters in front of it at least once \{1,\}
{M,n} does not require backslashes

Position anchoring
^
$
\<
\>

Group
() No backslash
\1,\2

Or
The meaning of a|b or
C|cat the whole left and right.
Cat or cat Error
C or Cat Pair

Grep-e ' C|cat ' text.txx

{3} repeated three times

Fgrep does not support regular expressions particularly fast

The use of common regexp in Linux environment