Detailed explanation of custom regular expression operator =~ in Swift

What are regular expressions?

Regular expressions, also known as regular expressions or conventional expressions (English: Regular Expression, commonly abbreviated as regex, regexp, or RE in code), are a concept in computer science. Regular expressions use a single string to describe and match a series of strings that conform to a certain syntax rule. Regular expressions are often used in many text editors to search for and replace text that matches a certain pattern.

Swift has not yet supported regular expressions at the language level, and there may not be many use cases for regular expressions in app development.

Encapsulation

In Cocoa, we can use NSRegularExpression to perform regular expression matching, so we encapsulate a RegularExpHelper based on NSRegularExpression to match whether a string conforms to a certain regular expression.

struct RegularExpHelper {
 let RegularExp: NSRegularExpression 
 init(_ pattern: String) throws {
  try RegularExp = NSRegularExpression(pattern: pattern, options: .caseInsensitive)
 } 
 func match(inpuut: String) -> Bool {
  let matches = RegularExp.matches(in: inpuut, options: [], range: NSMakeRange(0, inpuut.count))
  return matches.count > 0
 }
}

Custom =~

With the well-packaged RegularExpHelper, we can conveniently customize operators.

infix operator =~ : ATPrecedence
precedencegroup ATPrecedence {
 associativity: none
 higherThan: AdditionPrecedence
 lowerThan: MultiplicationPrecedence
}
func =~ (input: String, RegularExp: String) -> Bool {
 do {
  return try RegularExpHelper(RegularExp).match(input: input)
 }
  return false
 }
}

Operator definition

• infix indicates defining a middle operator (both inputs are before and after)
• prefix indicates defining a prefix operator (the input is before)
• postfix indicates defining a postfix operator (the input is after)

Associativity

That is the calculation order when multiple同类 operators appear in order

• left (from left to right order)
• right (from right to left order)
• none (the default is none, will not be combined again)

Precedence

• higherThan has a higher precedence than AdditionPrecedence this is the type of addition
• lowerThan has a lower precedence than MultiplicationPrecedence multiplication and division

Then we can use it

 if "[email protected]" =~ "^([a-z0-9_\\.-]+)@([\\da-z\\.-]+)\\.([a-z\\.]{2,6})$" {
  print("Meets email rules")
 }
  print("Does not meet email rules")
 }

Note

• Pay attention to the use of escape characters when using regular expression strings.
• Swift's operators cannot be defined in local scopes because operators need to be used in the global scope.
• There are many risks in overloading and customizing operators, please ask yourself many times whether you really need to do so before using them!

Summary

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