Swift中数字间可选下划线的正则表达式验证问题

Hey there! Let's break down why your current regex isn't working, then fix it up to perfectly fit your validation rules for Swift.

Why Your Regex Fails #

Your current pattern ([0-9]+_*[0-9]+)* has three key issues that make it fail your validation rules:

1. No full-string anchoring: Without ^ (start of string) and $ (end of string), the regex will match any valid substring within an invalid string. For example, abc123_def would be incorrectly marked as valid because it contains 123_def.
2. Allows consecutive underscores: The _* matches 0 or more underscores, so strings like 123__45 slip through even though they violate your "underscores only between numbers" rule.
3. Permits empty strings: The outer (...)* allows the entire group to repeat 0 times, which means an empty string would be considered valid—something you definitely don't want.

The Correct Regex for Your Rules #

You need a pattern that enforces:

• Only digits and underscores are allowed
• Underscores must be between digits (no leading/trailing underscores, no consecutive underscores)
• At least one digit is present

Here's the regex that checks all these boxes:

^[0-9]+(?:_[0-9]+)*$

Let's break this down:

• ^: Anchors the match to the start of the string
• [0-9]+: Matches one or more digits (ensures we start with a digit and have at least one number)
• (?:_[0-9]+)*: A non-capturing group that matches an underscore followed by one or more digits. The * means this group can repeat 0 or more times (allowing no underscores, or multiple valid underscore-separated digit groups)
• $: Anchors the match to the end of the string, ensuring the entire string is validated

Implementing in Swift #

Here's a clean Swift function that uses this regex to validate your strings. We'll use NSRegularExpression for reliable matching:

import Foundation

func isValidNumberString(_ input: String) -> Bool {
    let validationPattern = "^[0-9]+(?:_[0-9]+)*$"
    guard let regex = try? NSRegularExpression(pattern: validationPattern) else {
        // If regex fails to compile (shouldn't happen here), return false
        return false
    }
    let fullStringRange = NSRange(input.startIndex..., in: input)
    // Check if the entire string matches the pattern
    return regex.firstMatch(in: input, range: fullStringRange) != nil
}

// Test your examples to verify
let validStrings = ["123", "1_23", "123_4567_89_312_575"]
let invalidStrings = ["123_", "_123", "123__12", "_", "abc"]

print("Valid String Results:")
validStrings.forEach { print("\($0): \(isValidNumberString($0))") }

print("\nInvalid String Results:")
invalidStrings.forEach { print("\($0): \(isValidNumberString($0))") }

If you prefer a more concise version, you can use String's built-in range(of:options:) method:

func isValidNumberString(_ input: String) -> Bool {
    return input.range(of: "^[0-9]+(?:_[0-9]+)*$", options: [.regularExpression, .anchored]) != nil
}

Both implementations will correctly validate your test cases, rejecting all invalid examples and accepting the valid ones.

内容的提问来源于stack exchange，提问作者Pradeep Rajkumar