A detailed description of the Linux system grep and regular expressions

1 , grep: Searches for text based on patterns and displays lines of text that conform to patterns

Text Word wildcards regular the expression's meta-character combination to match the criteria

grep ' root '/etc/passwd Search the/etc/passwd file with the root character of the line displayed

- I.: Show only rows that are matched by a pattern

--color : Rows with pattern matching are displayed in color

- v : Only rows that are not matched by the pattern are displayed

- o : Displays only strings that are matched by the pattern

2 , regular expressions:

Metacharacters

.: indicates matching any single character

grep ' R.. T '/etc/passwd

[]: matches any single character within the specified range

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

Number of matches (greedy mode): (greedy mode means to match the pattern as long as possible)

* : Represents any time that matches its preceding character

a*b It means a appears, followed by a B a,b,ab,aab,acb,amnb at any one time, and the last three is not possible.

.* : Matches any character of any length

a.*b Represents a beginning B end of the middle is anything can a,b,ab,aab,acb,amnb front two is not possible

\?: represents 1 or 0 times the character that matches the preceding

\{m,n} : Indicates that the character before it matches at least m times up to n times (\ is used as escape to avoid {} being interpreted by the shell)

\{1,\} means at least once more. Unlimited \{0,3\} means up to 3 times less unlimited

grep ' a.\{1,3\}b ' file Represents a 1-character 2-character 3-character occurrence between A and b any of them can be, but at least 1 appear at most 3

Location anchoring:

^ : Anchor the beginning of the line any content after this character must appear at the beginning of the line

$ : Any content preceding this character in the end of the anchor line must appear at the end of the row

grep ' w$ '/etc/inittab search for lines ending in W in/etc/inittab

^$ : Blank line

grep^$/etc/inittab | Wc-l

\< or \b: The anchor word indicates that any character following it must appear as a word header

\> or \b: An anchor ending indicates that any character following it must appear as the tail of the word

\(\): Grouping

grep "\ (ab\) *" File Indicates that there is a line of AB in the file and no ab line appears.

3 , grep: A command to filter text using a pattern defined by a basic regular expression (default)

-i-v-o--color are basic regular Expressions

- e : Using an extended regular expression

4 , extending the regular expression:

Extending the meta-character of a regular expression

Character Matching:

.: indicates matching any single character

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

[]: matches any single character within the specified range

This article is from the "TT Melbourne TT" blog, reproduced please contact the author!

A detailed description of the Linux system grep and regular expressions