RegexBuilder.NET9

C# library for building .NET regular expressions with human-readable code.

Note: This is a fork of the original regex-builder by Yuriy Guts, updated to support .NET 9.

Quick links

• Getting Started — Install and create your first pattern in 5 minutes
• Common Patterns — Copy-paste ready production patterns (email, URL, phone, etc.)
• API Guide — Full reference for the Fluent and Classic APIs
• NuGet Package — Official NuGet package page for RegexBuilder.NET9

Table of Contents

• Installation
• Quick Start
• Real-World Example
• Documentation
• When to Use RegexBuilder
• Supported Features
• API Guide
• Advanced Usage
• Testing
• Development
• Contributing
• License

Overview

Just another day at the office—you write a .NET Regex like a boss, and suddenly realize that you need to declare a non-capturing group. Is it (?:pattern)? Or (?=pattern) (positive lookahead)? Or (?<=pattern) (positive lookbehind)?

"Aaargh! Where's that Regex cheat sheet?!"

The Solution: Inspired by Expression Trees in .NET, RegexBuilder provides a more verbose but infinitely more human-readable way of declaring regular expressions using fluent, chainable C# code instead of cryptic regex patterns.

When to Use RegexBuilder

• Complex expressions that might be frequently changed
• Team environments where regex readability matters for maintainability
• Building patterns programmatically with conditional logic
• When clarity is worth the modest performance cost of constructing the Regex object

Supported Features

The following elements are supported:

• Quantifiers
• Character escapes
• Character classes
• Anchors (atomic zero-width assertions)
• Grouping constructs
• Backreference constructs
• Alternation constructs
• Inline options and comments
• Substitution patterns - for use with Regex.Replace()

Installation

Install RegexBuilder.NET9 from NuGet using one of the following methods:

Via .NET CLI:

dotnet add package RegexBuilder.NET9 --version 1.1.1

Via PackageReference in .csproj:

<PackageReference Include="RegexBuilder.NET9" Version="1.1.1" />

You can also view the package page on NuGet: RegexBuilder.NET9 on NuGet

Quick Start

Here's a simple example to get you started. Let's build a regex to match email addresses:

var emailRegex = RegexBuilder.Build(
    RegexBuilder.Group(
        "localPart",
        RegexBuilder.CharacterSet(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore)
    ),
    RegexBuilder.Literal("@"),
    RegexBuilder.Group(
        "domain",
        RegexBuilder.Concatenate(
            RegexBuilder.CharacterSet(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore),
            RegexBuilder.Literal("."),
            RegexBuilder.CharacterSet(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore)
        )
    )
);

var match = emailRegex.Match("[email protected]");
if (match.Success)
{
    Console.WriteLine(match.Groups["localPart"].Value); // user
    Console.WriteLine(match.Groups["domain"].Value);    // example.com
}

Fluent API Quick Start (recommended)

Use the Fluent PatternBuilder for concise, chainable pattern creation. The following builds the same email validator:

var emailPattern = RegexBuilder.Pattern()
    .Start()
    .CharacterSet("a-zA-Z0-9._%+-", 1, null)
    .Literal("@")
    .CharacterSet("a-zA-Z0-9.-", 1, null)
    .Literal(".")
    .Letters(2, 6)
    .End()
    .Build();

var emailRegex2 = RegexBuilder.Build(emailPattern);
Console.WriteLine(emailRegex2.IsMatch("[email protected]")); // True

Real-World Example

Let's build a regex to capture href attributes from HTML hyperlinks. Compare the traditional approach:

Traditional regex (hard to read):

Regex hrefRegex = new Regex(
    "href\\s*=\\s*(?:[\"'](?<Target>[^\"']*)[\"']|(?<Target>\\S+))",
    RegexOptions.IgnoreCase
);

With RegexBuilder (self-documenting):

const string quotationMark = "\"";
Regex hrefRegex = RegexBuilder.Build(
    RegexOptions.IgnoreCase,
    RegexBuilder.Literal("href"),
    RegexBuilder.MetaCharacter(RegexMetaChars.WhiteSpace, RegexQuantifier.ZeroOrMore),
    RegexBuilder.Literal("="),
    RegexBuilder.MetaCharacter(RegexMetaChars.WhiteSpace, RegexQuantifier.ZeroOrMore),
    RegexBuilder.Alternate(
        RegexBuilder.Concatenate(
            RegexBuilder.NonEscapedLiteral(quotationMark),
            RegexBuilder.Group(
                "Target",
                RegexBuilder.NegativeCharacterSet(quotationMark, RegexQuantifier.ZeroOrMore)
            ),
            RegexBuilder.NonEscapedLiteral(quotationMark)
        ),
        RegexBuilder.Group(
            "Target",
            RegexBuilder.MetaCharacter(RegexMetaChars.NonwhiteSpace, RegexQuantifier.OneOrMore)
        )
    )
);

More examples can be found in CustomRegexTests.cs.

Documentation

Full documentation (including Getting Started, Common Patterns, and API Guide) is available in the docs/ folder and on the generated site.

• Local docs: docs/ (open docs/index.md)
• Generated site: https://somenoe.github.io/RegexBuilder.NET9/

API Guide

Fluent Pattern Builder

RegexBuilder now includes a Fluent Builder Pattern for composing complex regex patterns with improved ergonomics. The PatternBuilder class provides a chainable API for building patterns step-by-step.

Quick Example

// Build an ID pattern that matches either "ID-123" or "CODE-AB"
var pattern = RegexBuilder.Pattern()
    .Start()                          // ^ anchor
    .Literal("ID-")
    .Digits(3, 5)                     // \d{3,5}
    .Or(o => o.Literal("CODE-").Letters(2, 4))  // | CODE-[a-zA-Z]{2,4}
    .End()                            // $ anchor
    .Build();

var regex = RegexBuilder.Build(pattern);
Console.WriteLine(regex.IsMatch("ID-123"));   // True
Console.WriteLine(regex.IsMatch("CODE-AB"));  // True

PatternBuilder Methods

Anchors:

• Start() - Add start-of-line anchor ^
• End() - Add end-of-line anchor $

Patterns:

• Literal(string) - Literal text (auto-escaped)
• Digits(min, max) - Digit pattern \d with quantifiers
• Letters(min, max) - Letter pattern [a-zA-Z] with quantifiers
• Whitespace(min, max) - Whitespace pattern \s with quantifiers
• WordCharacter(min, max) - Word character pattern \w with quantifiers
• AnyCharacter(min, max) - Any character pattern . with quantifiers
• CharacterSet(charset, min, max) - Custom character set with quantifiers

Grouping:

• Group(action) - Capturing group (pattern)
• NonCapturingGroup(action) - Non-capturing group (?:pattern)

Operators:

• Or(action) - Alternation with another pattern via builder
• Or(node) - Alternation with an existing RegexNode
• Optional(action) - Optional pattern pattern?

Common Patterns:

• Email() - Email pattern
• Url() - URL pattern
• Pattern(node) - Add a custom RegexNode

Advanced Example

// Phone number pattern: [+1-]555-123-4567
var phonePattern = RegexBuilder.Pattern()
    .Optional(o => o.Literal("+1"))
    .Optional(o => o.Literal("-"))
    .Group(g => g.Digits(3))       // Area code
    .Optional(o => o.Literal("-"))
    .Group(g => g.Digits(3))       // Prefix
    .Optional(o => o.Literal("-"))
    .Group(g => g.Digits(4))       // Line number
    .Build();

var regex = RegexBuilder.Build(phonePattern);
Console.WriteLine(regex.IsMatch("555-123-4567"));     // True
Console.WriteLine(regex.IsMatch("+1-555-123-4567"));  // True
Console.WriteLine(regex.IsMatch("5551234567"));       // True

Core Classes

RegexBuilder has 4 main classes you'll work with:

1. RegexBuilder - Factory class for building regex patterns

   • Static methods that produce and combine different parts of a regular expression
   • Entry points: RegexBuilder.Build(...) and RegexBuilder.Pattern()

2. PatternBuilder - Fluent builder for composing patterns

   • Chainable methods for building complex patterns
   • Entry point: RegexBuilder.Pattern()

3. RegexQuantifier - Produces quantifiers for regex parts

   • Properties like ZeroOrMore, OneOrMore, Optional, ExactCount(n), Range(min, max), etc.

4. RegexMetaChars - Named constants for character classes

   • WordCharacter, NonwordCharacter, WhiteSpace, NonwhiteSpace, etc.
   • More readable than memorizing \w, \W, \s, \S

Basic Pattern Building

Start by calling RegexBuilder.Build() and pass the components you want to combine:

// Simple pattern
var pattern = RegexBuilder.Build(
    RegexBuilder.Literal("prefix-"),
    RegexBuilder.MetaCharacter(RegexMetaChars.Digit, RegexQuantifier.OneOrMore)
);
// Matches: "prefix-123", "prefix-42", etc.

// With options
var pattern = RegexBuilder.Build(
    RegexOptions.IgnoreCase,
    RegexBuilder.Literal("hello"),
    RegexBuilder.Literal(" "),
    RegexBuilder.Literal("world")
);

Grouping and Capturing

// Capturing group
var pattern = RegexBuilder.Build(
    RegexBuilder.Group("name",
        RegexBuilder.MetaCharacter(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore)
    )
);

// Non-capturing group
var pattern = RegexBuilder.Build(
    RegexBuilder.NonCapturingGroup(
        RegexBuilder.Alternate(
            RegexBuilder.Literal("cat"),
            RegexBuilder.Literal("dog")
        )
    )
);

Advanced Usage

Substitution Patterns

Use SubstitutionBuilder to create replacement patterns for Regex.Replace():

// Swap two words
var pattern = RegexBuilder.Build(
    RegexBuilder.Group("word1", RegexBuilder.MetaCharacter(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore)),
    RegexBuilder.Literal(" "),
    RegexBuilder.Group("word2", RegexBuilder.MetaCharacter(RegexMetaChars.WordCharacter, RegexQuantifier.OneOrMore))
);

var replacement = SubstitutionBuilder.Build(
    SubstitutionBuilder.Group("word2"),
    SubstitutionBuilder.Literal(" "),
    SubstitutionBuilder.Group("word1")
);

string result = pattern.Replace("hello world", replacement);
// result = "world hello"

Substitution Methods

The SubstitutionBuilder class supports all .NET substitution constructs:

• SubstitutionBuilder.Group(int) or Group(string) - Reference captured groups
• SubstitutionBuilder.WholeMatch() - Insert entire matched text ($&)
• SubstitutionBuilder.BeforeMatch() - Insert text before match ($`)
• SubstitutionBuilder.AfterMatch() - Insert text after match ($')
• SubstitutionBuilder.LastCapturedGroup() - Insert last captured group ($+)
• SubstitutionBuilder.EntireInput() - Insert entire input string ($_)
• SubstitutionBuilder.LiteralDollar() - Insert literal $ ($$)
• SubstitutionBuilder.Literal(string) - Insert literal text (auto-escapes $)

See SubstitutionBuilderTests.cs for more examples.

Testing

RegexBuilder uses MSTest for unit testing. The RegexBuilder.Tests project contains all unit tests.

Run tests from Visual Studio:

• Open the Test Explorer (Test > Test Explorer)
• Click "Run All Tests" or run specific test classes

Run tests from command line:

dotnet test src/RegexBuilder.slnx

Test project structure:

• RegexBuilder.Tests/ - All unit test files
• RegexBuilder.Examples/ - Executable examples demonstrating all README code samples

Development

Format code (requires Prettier and dotnet format):

make format
# or manually:
npx prettier --write .
dotnet format src

Build the project:

dotnet build src/RegexBuilder.slnx

Pack as NuGet package:

dotnet pack src/RegexBuilder/RegexBuilder.csproj

Contributing

Contributions are welcome! Please:

1. Create a feature branch from master
2. Add tests for any new functionality
3. Run make format to format code
4. Submit a pull request with a clear description

License

The source code is licensed under The MIT License.