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Regular expression

Regular Expression is an important concept in computer science. It uses a mathematical algorithm to solve the problems of text retrieval and matching in computer programs. Regular Expression language is a language specially used for string processing. It is supported in many languages and c# is no exception

Two functions:

Retrieve: Retrieve the desired part of a string

Match: Determines whether a string conforms to the filtering logic of a regular expression

Common usage: Check whether the password or email format entered by a user is valid

A regular expression is a text pattern consisting of ordinary characters and special characters. Special characters are metacharacters

The \ escape character is used to represent \

The @ symbol, which adds the @ character to a regular expression to indicate that it does not allow the compiler to parse the escape characters in the regular expression

locator

^ Matches the start of the string

$matches the end position of the string

\b Matches the boundary of a word

\B Matches non-word boundaries

Example: Match start and end

string str = "World"; Replace(STR, "^", "Hello "); // Match the starting position with the specified string string res = regx.replace (STR, "^", "Hello "); Replace(STR, "$", "!" ); / / o World!Copy the code

Matches any character except newline

\w Matches letters, digits, underscores, and Chinese characters (upper and lower case letters, digits from 0 to 9, and underscores).

\W \W complement (except “upper and lower case letters, 0-9 digits, underscore _”)

\s Matches any whitespace characters (including newline /n, carriage return /r, TAB /t, vertical TAB /v, feed /f)

Complement of \S \S (except for the characters defined by \S)

\d Matching digits (0-9 digits)

\D denotes the complement of \D (except for 0-9 digits)

Example: Only numbers are allowed

Regex.IsMatch(input, @"^\d*$");
Copy the code

[ABC] matches a, B, or C

[a-c] matches the characters between A and C

[^x] matches any character other than x

Example: Replace any character except ace with *

string str = "abcdef"; string pattern = @"[^ace]"; Replace(STR, pattern, "*"); string res = regx.replace (STR, pattern, "*"); // replace any character except ace with * // The result is a* C *e*Copy the code

qualifiers

* indicates that zero or multiple occurrences occur

+ indicates one or more occurrences

? Indicates 0 or 1 occurrence

{n} represents n occurrences, and n represents a non-negative integer

{n,} indicates at least n occurrences

{n,m} indicates n to m occurrences

Example: verify whether the entered QQ number is valid (note: QQ number is 5-12 digits)

string qq1 = "123", qq2 = "397458006",qq3="abc123456"; String pattern = @ "^ \ d {5, 12} $"; bool isMatch1 = Regex.IsMatch(qq1, pattern); //false bool isMatch2 = Regex.IsMatch(qq2, pattern); //true bool isMatch3 = Regex.IsMatch(qq3, pattern); //falseCopy the code

Choose a match

| characters, the two matching conditions by using the Boolean or operation

Example: Find numbers or letters

String STR = "123 "; string pattern = @"\d|[a-z]"; MatchCollection col = Regex.Matches(str, pattern); foreach (Match match in col) { Console.WriteLine(match); }Copy the code

The results for

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grouping

Use parentheses ()

Example: Replace a single character twice with **

string str = "aabbaaacccaaaa"; string pattern = @"a{2}"; string res = Regex.Replace(str, pattern, "**"); **bb**accc****Copy the code

Example: Verify the IP4 address

string pattern = @"^(((2[0-4]\d|25[0-5]|[01]? \d\d?) \.) {3}(2[0-4]\d|25[0-5]|[01]? \d\d?) ) $";Copy the code

Related links:

Learning regular expressions

Common regular expressions

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