1. What is a regular expression

Regular expressions, also known as normal notation, regular notation (English: Regular Expression, often abbreviated regex, regexp, or RE in code), are a concept in computer science.

Regular expressions use a single string to describe and match strings in series 1 that conform to certain syntactic rules.

In many text editors, regular expressions are used to retrieve and replace text that fits a pattern.

2. Character composition of regular expressions

Ordinary characters [a~z], special characters (called “metacharacters”)

3. Support

Almost all programming languages support regular expressions, such as OC, swift, java, c#, python, js, and so on

Regular expressions can be used for retrieval in many text editors, and Xcode also supports regular expressions!

Use 4.

matching

(pattern) matches pattern and gets the 1 match, which can be obtained from the resulting Matches set

A collection of

[xyz] character set (x|, |, y|, |, z) [a-z] character range a-z [a-zA-Z] character range a-z A-Z [^xyz] negative character set (any character except xyz) [^ a-z] negative character range [a-d][m-p] union (a to d or m to p)

Common metacharacter

Matches any character other than a newline

\w to match letters or Numbers or underline or Chinese characters [a-zA-Z_0-9] \s matches any blank character (space, TAB\t, carriage return \r \n) \d matching Numbers [0-9] ^a matches the a character at the beginning of the string a$matches the end of the a character string \bw matches the start or end of a word with w characters

Often use antisense characters

\W matches any character other than letter, number, underscore, Chinese character [^\w] \S matches any character that is not a blank [^\s] \D matches any non-numeric character [^0-9] \Ba matches a characters that are not at the beginning or end of a word [^a] matches any character other than a [^aeiou] matches any character other than the letters aeiou

Common qualifier

w*oo zero or more times w+oo repeat 1 or more times w & # 63; oo zero or one repeat w{n} w repeat n times w{n,} w repeat n or more w{n,m} w repeat n to m times

Greed and laziness

* & # 63; Repeat as many times as possible, but with as few repetitions as possible *+ repeat 1 or more times, but as little as possible The & # 63; The & # 63; Repeat 0 or 1 times, but as little as possible w} {1, 2 & # 63; Repeat 1 or 2 times, but as little as possible ww {1} & # 63; Repeat more than once, but as little as possible

Example 5.

//  judge QQ Number (routine judgment)
 fileprivate func checkIsQQNumber(str:String) ->Bool {
   // 1. Judge whether 0 At the beginning
   if str.hasPrefix("0"){
     return false
   }
   // 2. Judge whether 5~15 position
   if str.characters.count < 5 || str.characters.count > 15{
     return false
   }
   // 3. Determine if it's all Numbers
   for c in str.characters{
     if c < "0" || c > "9"{
       return false
     }
   }
   return true
 }
//  Regex determines the phone number
 fileprivate func checkPhoneNumber(str:String)->Bool {
   let pattern = "1[3578]\\d{9}"
   let regex = try! NSRegularExpression(pattern: pattern, options: NSRegularExpression.Options(rawValue:0))
   let res = regex.matches(in: str, options: NSRegularExpression.MatchingOptions(rawValue:0), range: NSMakeRange(0, str.characters.count))
   if res.count > 0 {
     return true
   }
   return false
 }

conclusion