Using GeneratedRegex (Source Generator)

Using GeneratedRegex (Source Generator)

Use GeneratedRegexAttribute for compile-time regex generation instead of runtime new Regex().

Why GeneratedRegex?

Aspect	Runtime Regex	GeneratedRegex
Compilation	Runtime	Compile-time
Performance	Slower first match	Pre-compiled, faster
AOT Support	Limited	Full support
Memory	Allocates at runtime	No runtime allocation
.NET Version	All	.NET 7+

---

Basic Pattern

public partial class EmailValidator
{
    [GeneratedRegex(@"^[\w\.-]+@[\w\.-]+\.\w+$", RegexOptions.IgnoreCase)]
    private static partial Regex EmailPattern();

    public bool IsValidEmail(string email)
    {
        return EmailPattern().IsMatch(email);
    }
}

Requirements:

• Class must be partial
• Method must be static partial returning Regex
• .NET 7 or later

---

Common Patterns

Email Validation

public partial class ValidationPatterns
{
    [GeneratedRegex(@"^[\w\.-]+@[\w\.-]+\.\w+$", RegexOptions.IgnoreCase)]
    public static partial Regex Email();
}

Phone Number

[GeneratedRegex(@"^\d{3}-\d{3,4}-\d{4}$")]
public static partial Regex PhoneNumber();

URL Pattern

[GeneratedRegex(@"^https?://[\w\.-]+(?:/[\w\.-]*)*$", RegexOptions.IgnoreCase)]
public static partial Regex Url();

Whitespace Normalization

[GeneratedRegex(@"\s+")]
private static partial Regex WhitespacePattern();

public string NormalizeWhitespace(string input)
{
    return WhitespacePattern().Replace(input, " ");
}

---

Migration from Runtime Regex

Before (Runtime)

// Anti-pattern: Runtime compilation
public class Validator
{
    private static readonly Regex _emailRegex =
        new(@"^[\w\.-]+@[\w\.-]+\.\w+$", RegexOptions.Compiled);

    public bool IsValid(string email)
    {
        return _emailRegex.IsMatch(email);
    }
}

After (Source Generator)

// Best practice: Compile-time generation
public partial class Validator
{
    [GeneratedRegex(@"^[\w\.-]+@[\w\.-]+\.\w+$")]
    private static partial Regex EmailPattern();

    public bool IsValid(string email)
    {
        return EmailPattern().IsMatch(email);
    }
}

---

RegexOptions

// Multiple options
[GeneratedRegex(@"pattern", RegexOptions.IgnoreCase | RegexOptions.Multiline)]
private static partial Regex MultiOptionPattern();

// With timeout (for untrusted input)
[GeneratedRegex(@"complex.*pattern", RegexOptions.None, matchTimeoutMilliseconds: 1000)]
private static partial Regex SafePattern();

---

WPF Validation Example

public partial class FormViewModel : ObservableValidator
{
    [GeneratedRegex(@"^[\w\.-]+@[\w\.-]+\.\w+$", RegexOptions.IgnoreCase)]
    private static partial Regex EmailPattern();

    [GeneratedRegex(@"^\d{3}-\d{3,4}-\d{4}$")]
    private static partial Regex PhonePattern();

    [CustomValidation(typeof(FormViewModel), nameof(ValidateEmail))]
    [ObservableProperty] private string _email = string.Empty;

    [CustomValidation(typeof(FormViewModel), nameof(ValidatePhone))]
    [ObservableProperty] private string _phone = string.Empty;

    public static ValidationResult? ValidateEmail(string email, ValidationContext context)
    {
        if (string.IsNullOrEmpty(email))
            return new ValidationResult("Email is required");

        if (!EmailPattern().IsMatch(email))
            return new ValidationResult("Invalid email format");

        return ValidationResult.Success;
    }

    public static ValidationResult? ValidatePhone(string phone, ValidationContext context)
    {
        if (string.IsNullOrEmpty(phone))
            return ValidationResult.Success; // Optional field

        if (!PhonePattern().IsMatch(phone))
            return new ValidationResult("Format: 000-0000-0000");

        return ValidationResult.Success;
    }
}

---

GlobalUsings.cs

global using System.Text.RegularExpressions;

---

Checklist

• Class is marked as partial
• Method is static partial returning Regex
• Pattern string is valid regex
• Target framework is .NET 7+
• Consider RegexOptions for case sensitivity
• Add timeout for untrusted input patterns