Regular expressions (matches under the grep command)

Basic Regular expression: REGular expression (REGEXP)

One, meta-character:

1. . : matches any single character;

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

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

PS : Common character sets

[:d igit:]--Digital Collection

[: lower:]--Small Letter Set

[: upper:]--Caps Collection

[: space:]--Space

[: alpha:]--All letters set

[: alnum:]--All letters and numbers set

Second, the number of matches:

1. *: Match the characters in front of it any time;

2. . *: Matches any character of any length;

3. \? : matches the characters in front of it at least once;

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

PS : The regular expression defaults to working in greedy mode, which matches the maximum length string for an expression.

Three, the position anchor nails:

1. ^: Anchor at the beginning of the line, the content after this character must appear at the top;

2. $: Anchor Line end, this character before the content must appear at the end of the row;

3. ^$: Blank line (the line at which the anchor is just beginning to end);

4. \< or \b: Anchor word, any character behind it must appear as the first word (non-English word);

5. \> or \b: anchor ending, any character in front of which must appear as the tail of a word (non-English word).

   For example,:\<root\> matches the word root.

   
   

Four, group:

1. \ (\): Matches the grouped content, matching the contents of the parentheses as a whole.

   For example: \ (ab\) * means matching "ab" any time.

2. A back reference (someone also referred to as a forward reference): Used with grouping.

   \1--refers to the first opening parenthesis and all content contained in the corresponding closing parenthesis (whether or not nested in parentheses)

   \2--refers to the second opening parenthesis and all the contents of the corresponding closing parenthesis (IBID.)

   \3 —— ... (An analogy) ...

   For example, the following three lines of content:

   He like his liker.

   She Love her lover.

   He like her lover.

   Expression: L.. E can match each line;

   Expression: \ (L.. e\). *\1 can only match the first two lines.

   
   

Extended Regular expression: (differs from basic regular expression)

1. ? : matches the number of characters in front of it at most once (note: there is no \ in front of it);

2. +: Matches the number of characters before it at least once (note: The basic regular expression does not have this metacharacters);
   

3. {M,n}: Number of times match, usage with basic regular expression (no need to use \ in front of curly braces);

4. (): Grouping, the real grouping (no need to use \), also support the back reference;

5. | Or For example: A|b matches A or b,c|cat matches C or cat (matches the entire left or right).
   

Over.

This article is from the "stupid Bird Learn to fly" blog, please be sure to keep this source http://yshun.blog.51cto.com/9570549/1718837

Regular expressions (matches under the grep command)