Introduction to Regular expressions

Regular expression definition: A collection of characters or special string patterns.

Function: Searches for text based on the pattern and displays lines of text that conform to the pattern.

Pattern (pattern): Text word wildcards regular the expression's meta-character combination to match the condition

#正则表达式就是里面有一些元字符, these characters do not represent the meaning of itself, but the meaning of a wildcard.

Note: Regular expressions work in greedy mode by default

Grep:

#文本搜索过滤工具, support for basic regular expressions

Synopsis:grep [OPTIONS] PATTERN [FILE ...]

Options:

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

-i:igmorecase, ignoring the case of characters;

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

-v:--invert-match: Reverse display;

-N: Output line number

-C: Count the occurrences of the string that is matched to

-W: Exact match in characters

-E:--ENTENDED-REGEXP: Supports extended regular expression meta-characters;

-q:--quiet,--silent; silent mode, do not output any information;

-A #:after, after # line

-B #:before, front # line

-c #:context, front and back # lines

Basic regular Expression meta-characters:

Character Matching:

. : matches any single character;

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

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

[:d Igit:]

[: Alpha:]

[: Upper:]

[: Lower:]

[: Space:]

[: Alnum:]

[:p UNCT:]

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 one or more times, that is, the preceding character appears at least once;

{m}: matches the preceding character m times;

{M,n}: matches the characters preceding it at least m times, up to n times;

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$: Using PATTERN to match the whole line;

^$: blank line;

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

< b: The first anchor of the word, used for the leftmost side of the word;

> B: The ending anchor for the rightmost side of the word;

<pattern>: Exact match of complete words;

Grouping and referencing

(): Bind one or more characters together and treat as a whole

(XY) *ab

Note:

The contents of the pattern in the grouping brackets are automatically recorded in the internal variables by the regular expression engine, which are:

1: The pattern from the left side, the first opening parenthesis and the matching closing parenthesis, matches the character of the pattern;

2: The pattern from the left side, the second opening parenthesis and the matching closing parenthesis, matches the character of the pattern;

3: ...

...

* Back reference: Refers to the character to which the pattern in the preceding grouping brackets matches

Extending the meta-character of a regular expression

Character Matching:

.：

[]:

[^]:

Number of matches:

*: Any time

？ : 0 or 1 times,

+: Its first character at least once;

{m}: the characters before the period m times;

{M,n}: At least m times, up to n times;

Location anchoring:

^: Anchor at the beginning of the line

$: End of line anchoring

<,b: The first anchor of the word

>,b: Ending anchoring

Grouping and referencing:

(): grouping; the characters that match the pattern in parentheses are recorded in the internal variables of the regular expression engine;

Back To reference: ...

Or:

A|b:

C|cat:c or Cat

(c| C) At:cat or cat

Introduction to Regular expressions