Why do you need regular expressions?
complex processing of text
Advantages and uses of regular expressions?
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A powerful and flexible text processing tool
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Most programming languages, databases, text editors, and development environments support regular expressions
regular expression syntax
1. Ordinary characters
Letters, numbers, Chinese characters, underscores, and punctuation marks without special definitions are all "common characters". Ordinary characters in the expression, when matching a string, match the same character.
2. Simple escape characters
Special fields are used for their own purposes. Special characters cannot be used directly. They need to be escaped before they can be used. In the last two lines of the table, "/" needs to be added to express their own characters after escaping.
3. Standard character set
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An expression capable of matching 'multiple characters'
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Note that case is sensitive, uppercase means the opposite
4. Custom character set
[]Square bracket matching mode, which can match any character in the square brackets
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The special symbols of regular expressions, if they are included in square brackets, lose their special meaning, except for ^, -
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The standard character set, except for the decimal point, if it is enclosed in square brackets, the custom character set will include the set. For example: [\d.\-+] will match: numbers, decimal points, +, -
5. Quantifier
Special symbols that modify the number of matches
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Greedy mode in the number of matches (the more characters to match, the better, this mode is used by default)
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Non-greedy mode in the number of matches (the fewer matching characters the better, add a "?" after the special symbol that modifies the number of matches)
6. Character boundaries
(This group of tags matches not characters but positions, positions that meet certain conditions)
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If ^ and $ are together, it must be an exact match.
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\b matches such a position: the preceding character and the following character are not all \w
Regular expression matching pattern
IGNORECASE ignore case pattern
Case is ignored when matching.
By default, regular expressions are case sensitive.
SINGLELINE single line mode
The entire text is regarded as a string with only one beginning and one end.
Make the decimal point "." match any character including a newline character (\n).
MULTILINEMultiline mode
Each line is a string with a beginning and an end.
After specifying MULTILINE, if you need to match only the beginning and end of the string, you can use \A and \Z
7. Selectors and grouping
8. Backreference (\nnn)
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Each pair of () will be assigned a number, and the captures using () will be automatically numbered from 1 according to the order of the opening brackets.
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By means of backreferences, references can be made to strings already captured by the group.
9. Pre-search (zero-width assertion)
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Only sub-expression matching is performed, the matching content is not included in the final matching result, and it is zero-width
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This location should meet a certain condition. Determine whether the characters before and after the current position meet the specified conditions, but do not match the characters before and after. is a match on position.
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During the regular expression matching process, if the subexpression matches the character content instead of the position and is saved in the final matching result, then the subexpression is considered to be character-occupied; if the subexpression matches only is the position, or the matched content is not saved into the final matching result, then the subexpression is considered to be zero-width. Occupying characters or zero width depends on whether the matched content is saved in the final matching result.
List of common regular expressions:
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