The essential "grep trio" and regular expressions in Linux

Text processing on Linux "threesome"

grep, Egrep, Fgrep: Text Search tool; Search operations on a given text based on "pattern";

Regular Expressions: Regular expression, REGEX, 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 denote the function of control or distribution;

Divided into two categories:

Basic Regular Expressions: BRE

Extended Regular expression: ERE

grep family:

grep: Supports the use of basic regular expressions;

Egrep: Supports the use of extended regular expressions;

Fgrep: The use of regular expressions is not supported;

grep command:

Function: Text Search tool, according to the user specified "pattern (filter)" to match the target text line by row to check, print out the qualifying line;

Common options:

--color=auto: Color The matching text to highlight;

-I: ignore character case;

-O: Displays only the text that matches to itself;

-V,--invert-match: reverse match;

-E: Support for extended regular expressions;

-Q,--quiet,--silient: Silent mode, do not output any information;

Basic regular Expression meta-characters:

Character Matching:

.: matches any single character;

[]: matches any single character within the range;

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

[:d Igit:],[:lower:], [: Upper:], [: Alpha:], [: Alnum:], [: Space:], [: Blank:], [:p UNCT:]

Number of matches:

Used to specify the number of occurrences of the character to be preceded by a limit to how many times the preceding characters appear, the default work in greedy mode;

*: matches the preceding character any time (0,1 or more);

. *: Any character of any length;

\+: Matches the preceding character at least 1 times;

\?: matches the previous 0 or 1 times;

\{b\}: The preceding character appears B, B is a non-negative integer;

\{b,d\}: The preceding character appears B, B is a non-negative integer, and a maximum of B times, at least D

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

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

Location anchoring:

Restricts the use of pattern search text, which restricts the text that the pattern matches to where it appears only in the target text;

^: Anchor at the beginning of the line; for the leftmost side of the pattern, ^pattern

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

^pattern$: To make PATTERN exactly match a whole line;

^$: Empty line;

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

\< or \b: The initial anchor for the left side of the word pattern, formatted as \<pattern, \bpattern

\> or \b: The ending anchor for the right side of the word pattern, formatted as PATTERN\> pattern\b

\<pattern\>: Word anchoring;

Grouping and referencing:

\ (pattern\): The character matching this PATTERN is treated as a non-infringing whole;

Note: The patterns in the grouping brackets match the characters that are automatically recorded in the internal variables by the regular expression engine, which are \1, \2, \3, ...

Back reference: Refers to the string that matches the pattern in the preceding parentheses;

Two common options:

-E,--extended-regexp: supports the use of extended regular expressions

-F,--fixed-strings: Supports the use of fixed strings, does not support regular expressions, and is equivalent to Fgrep;

-G,--basic-regexp: Supports the use of basic regular expressions;

-P,--perl-regexp: supports the use of pcre regular expressions;

-E pattern,--regexp=pattern: multi-mode mechanism;

-F file,--file=file:file a text file containing a pattern for each line, the grep script;

Egrep

The grep command that supports the use of extended regular expressions is equivalent to GREP-E;

Extend the metacharacters of regular expressions:

Character Matching:

.: Any single character

[]: Any single character within the range

[^]: Any single character outside the range

Number of matches:

*: any time;

?: 0 or 1 times;

+:1 or multiple times;

{B}: Match B-time;

{b,d}: At least B times, at most d times;

{0,d}: up to D times

{b,}: at least B times

Location anchoring:

^: Beginning of the line

$: End of line

\<, \b: The head of the word

\>, \b: suffix

Grouping and referencing:

(pattern): a grouping in which the pattern in parentheses matches to a character that is recorded in a variable inside the hermetical expression engine;

Or:

A|b:a or B

C|cat: Indicates C or cat

(C|C) at: Indicates Cat or cat

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The essential "grep trio" and regular expressions in Linux