Linux Regular expressions

The composition of the regular expression:

1. General characters: characters with no special meaning
2. Special characters (meta characters): metacharacters, which have special meanings in regular expressions

Common meta-characters in regular expressions

1. POSIX Bre with both meta characters in ere
   1. \ usually used to turn on or off special meanings of subsequent characters, such as \ (... \) with \{...\};
   2. . Matches any single character (except null);
   3. * Match any number or no single character before it, for example:. Represents any one character, then. * matches any length of any character;
   4. ^ matches the regular expression immediately thereafter, the Bre only has a special meaning at the beginning of the regular expression, and ere in any position has a special meaning;
   5. $ matches the preceding regular expression, either at the string or at the end. The BRE has a special meaning only at the end of the regular expression, and Ere has a special meaning in any position;
   6. [] matches any one of the characters in the square brackets, where the hyphen (-) is the range of consecutive characters, and the ^ symbol appears in the first position of the square brackets, indicating that the match is not any character in the list;
2. Only characters in POSIX Bre
   1. \{n,m\}: An interval expression that matches the number of occurrences of a single character before it. \{n\} refers to reproducing n times; \{n,m\} refers to reproducing N to M times;
   2. \ (\): Reserve space to store up to 9 independent sub-patterns in a single pattern. such as \ (ab\). *\1: Refers to two occurrences of a matched AB combination, with any number of characters in the middle.
   3. \ n: Repeats the pattern of the nth sub-pattern in the \ (\) parentheses;
3. POSIX ere there are characters
   1. {n,m} is the same as in the BRE
   2. + one or more extensions that match the preceding regular expression
   3. ? Match 0 or one extension of the preceding regular expression
   4. | Match | Regular expression before or after a symbol
   5. () match () a regular expression group enclosed
4. Character
   The identity character set has the following types:

[::alnum]: Span style= "font-family: Arial" > numeric characters	[:d igit:]: Span style= "font-family: Arial" > numeric characters	[:p UNCT:]: Span style= "font-family: Arial" > punctuation character
[:alpha:]: Span style= "font-family: Arial" > alphabetic character	[:graph:]: Span style= "font-family: Arial" > non-whitespace character	[:space:]: space character
[:blank:]: Span style= "font-family: Arial" > spaces and positioning characters	[:lower:]: Span style= "font-family: Arial" > lowercase alphabetic characters	[:upper:]: Span style= "font-family: Arial" > capital letter character
[:cntrl:]: Span style= "font-family: Arial" > control character	[:p rint:]: Span style= "font-family: Arial" > The characters that can be displayed	[:xdigit:]:
[:: Alnum]: Numeric characters	[:d igit:]: Numeric characters	[:p UNCT:]: Punctuation characters

Case One

The contents of the original file Url.txt are as follows:

Www.baidu.com

Http://www.baidu.com

Https://www.baidu.com

Http://wwwbaiducom

Baidu.com

Baidu

Requirement: Match begins with http or HTTPS, followed by: and contains a string of.
Bre match:

Grep ' ^https\{0,1\}:.*\. * ' Url.txt

Ere match:
Grep–e ' ^https?:.*\. * ' Url.txt

Case Two

Email match

The contents of the original file Email.txt are as follows:

[Email protected]

[Email protected]

[Email protected]

[Email protected]

@ @baidu. com

Requirement: A match begins with a letter or a number, or an underscore, followed by an @ followed by multiple alphanumeric or underlined, with one.

Grep ' ^[[:alpha:][:d igit:]_]*@[[:alpha:][:d igit:]]*\. * ' Email.txt

Note: This article refers to http://www.jb51.net/article/42989.htm

Linux Regular expressions