Regular Expressions of Linux

1. Text Processing tools (grep, sed, awk)

grep: Text Filter tool (mode: pattern);

Sed:stream Editor, stream editors, text editors;

The implementation on Awk:linux is Gwak, Text Report Generator (formatted text);

Regular expressions are used in all three of these tools.

2. Regular Expressions: (Regual expression,regexp): A pattern written by a class of special characters and text characters, some of which do not represent their literal meaning, but are used to represent a function of control or wildcard.

Regular expression classification: The basic regular Expression Bre, extended regular expression ere. (The difference between the two: meta-characters are different)

Regular expression meta-characters:

grep: Searches each line of the file thoroughly and matches the pattern (filter condition) to output matching rows .

Pattern: The filter condition written by metacharacters and text characters of regular expressions.

Regular expression engine:

grep usage:

grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-E PATTERN |-f file] [FILE ...]

OPTIONS:

--color=auto: Color highlighting of matched text;

-i:ignorecase; ignore character case;

-O: Displays only the matching string itself, and the default display is the matching line;

-v:--invert-match; Displays rows that cannot be matched to a pattern;

-E: Supports the use of extended regular expressions;

-q:--quit,--silent; do not output anything

-A #: show the after # lines that match to

-B #: Displays the first # lines matched to before

-C #: Displays the front and back # lines that match to; context

Basic Regular Expression metacharacters: (classification: Character matching: Number of matches, location anchoring, grouping, and referencing)

Character Matching:

.: Matches any single character (point number)

[]: matches any single character within the specified range;

[^]: matches any single character outside the specified range;

[[:d Igit:]]; [[: Lower:]]; [[: Upper:]]; [[: Alpha:]]; [[: Alnum:]]; [[:p UNCT:]]; [[: Space:]]

[[email protected] ~]# grep r[[:alpha:]][[:alpha:]]t/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Operator:x:11:0:operator:/root:/sbin/nologin

[Email protected] ~]#

Number of matches: used to limit the number of occurrences of the preceding character, after the character to specify the number of occurrences.

*: Match its preceding characters any time: 0, 1, multiple times;

. *: Matches any character of any length;

\?: matches the preceding character 0 or 1 times, that is, the preceding character is optional;

\+: Matches the preceding character 1 or more times, i.e. the preceding character must appear at least 1 times;

\{m\}: Matches the preceding character m times;

\{m,n\}: Matches its preceding character at least m times, up to n times;

\{0,n\}: Up to n times;

\{m,\}: at least m times;

[[email protected] ~]# grep ro*t/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Operator:x:11:0:operator:/root:/sbin/nologin

Abrt:x:173:173::/etc/abrt:/sbin/nologin

Rtkit:x:172:172:realtimekit:/proc:/sbin/nologin

Pkiuser:x:17:17:certificate System:/usr/share/pki:/sbin/nologin

[Email protected] ~]#

Location anchoring:

^: Anchor at the beginning of the line: for the leftmost mode;

$: End of line anchoring: for the right-most side of the pattern;

^pattern$: Used for PATTERN to match whole line;

^$: blank line;

^[[:space:]]*$: Blank lines or lines that contain white space characters;

Word: A continuous character (string) consisting of a non-special character is called a word;

\< or \b: The first anchor of the word, the left side of the word pattern;

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

\<pattern\>: matches complete words;

[[email protected] ~]# grep "\<root"/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Operator:x:11:0:operator:/root:/sbin/nologin

[[email protected] ~]# grep "\broot"/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Operator:x:11:0:operator:/root:/sbin/nologin

[[email protected] ~]# grep "root\>"/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Operator:x:11:0:operator:/root:/sbin/nologin

[Email protected] ~]#

[[email protected] ~]# grep "^root"/etc/passwd

Root:x:0:0:root:/root:/bin/bash

[[email protected] ~]# grep "root$"/etc/passwd

[Email protected] ~]#

Grouping and referencing:

Such as:

[[email protected] ~]# grep uuid/etc/fstab

Uuid=47c2d0d2-2c32-4a36-89e7-420739594a95/boot XFS Defaults 0 0

[Email protected] ~]#

Regular Expressions of Linux