CA2028: Avoid redundant Regex.IsMatch before Regex.Match

[danmoseley]

Implements CA2028: Avoid redundant Regex.IsMatch before Regex.Match.

Closes dotnet/runtime#111239
Supersedes #51214 (abandoned — Copilot was not converging at the time)

Pattern detected

// Before (local declaration)
if (Regex.IsMatch(input, pattern))
{
    Match m = Regex.Match(input, pattern);
    // use m
}

// Before (pre-declared variable)
Match m = null;
if (Regex.IsMatch(input, pattern))
{
    m = Regex.Match(input, pattern);
    // use m
}

// After (both cases)
if (Regex.Match(input, pattern) is { Success: true } m)
{
    // use m
}

What's included

• Analyzer (IOperation-based, language-agnostic): registers on IConditionalOperation, validates operand equivalence (restricted to stable sources: locals, parameters, constants, readonly/const fields), tracks intervening writes including ref/out argument mutations, and reports diagnostic with additional location on the Match call.
• C# fixer with two paths:
  • Local declaration: Match m = Regex.Match(...) inside the if body — replaces with is pattern.
  • Pre-declared variable assignment: Match m = null; if (...) { m = Regex.Match(...); ... } — removes the declaration and assignment, replaces with is pattern. Includes safety checks: variable must not be referenced after the if statement, initializer must be absent/null/default.
• 82 tests covering: all static/instance overloads (including timeout), diagnostic+fix, diagnostic-only (no fix offered), no-diagnostic, VB smoke tests, real-world patterns found via GitHub code search (else-if chains, foreach loops, const fields), parenthesized arguments, pre-declared variable assignment patterns, edge cases (intervening writes, mutable receivers, property/method arguments, ref/out mutations, readonly-field receiver reassignment, span overloads with Net70 reference assemblies).

Addresses all feedback from #51214

All 15 review comments from @stephentoub on the abandoned PR are addressed:

• Resource strings don't mention "pattern matching"
• Inverted case (!IsMatch guard) intentionally not supported per reviewer guidance — adds too much complexity
• Full intervening-write analysis prevents false positives from variable reassignment
• Instance receiver equivalence validated with same rules as arguments
• No VB fixer (VB analyzer works, fixer is EmptyCodeFixProvider)
• No unnecessary wrapper methods; context.Diagnostics[0] not inlined because it's used 4x
• Extensive test coverage (82 tests vs ~10 in original)

Design decisions

• Rule ID CA2028 — CA2027 is already taken by DoNotUseNonCancelableTaskDelayWithWhenAny
• Category: Reliability, severity: IdeSuggestion
• Operand equivalence restricted to stable sources — properties, method calls, mutable fields are rejected
• Does not recurse into lambdas, local functions, or nested blocks for Match call search
• AddMutableSymbol recurses into IFieldReferenceOperation.Instance to handle obj.ReadonlyField receiver patterns
• Span overloads (ReadOnlySpan) intentionally not flagged — IsMatch returns bool but there's no corresponding Match overload returning Match
• Named arguments matched by Parameter.Ordinal (not array index) for correctness
• WalkDownParentheses() applied consistently before WalkDownConversion() in all operand/symbol tracking paths

[Copilot]

Pull request overview

Adds the CA2028 analyzer and C# code fix to detect and fix redundant Regex.IsMatch(...) guards immediately followed by Regex.Match(...) with equivalent operands, reducing duplicated regex evaluation work across C# and VB (analyzer) with a C#-only fixer.

Changes:

• Introduces CA2028 analyzer (IOperation-based) to detect redundant IsMatch→Match patterns with operand equivalence + intervening-write checks.
• Adds a C# code fix to rewrite the pattern into Regex.Match(...) is { Success: true } m when safe.
• Adds extensive unit tests plus rule metadata/resource updates (resx/xlf), documentation, and SARIF entries.

Show a summary per file

File	Description
src/Microsoft.CodeAnalysis.NetAnalyzers/tests/Microsoft.CodeAnalysis.NetAnalyzers.UnitTests/Microsoft.NetCore.Analyzers/Runtime/AvoidRedundantRegexIsMatchBeforeMatchTests.cs	Adds CA2028 analyzer/fixer coverage across many positive/negative/edge scenarios (incl. VB analyzer smoke tests).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/AvoidRedundantRegexIsMatchBeforeMatch.cs	New CA2028 analyzer implementation for redundant Regex.IsMatch before Regex.Match.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.CSharp.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/CSharpAvoidRedundantRegexIsMatchBeforeMatch.Fixer.cs	New C# code fix provider for CA2028 using property-pattern matching.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/MicrosoftNetCoreAnalyzersResources.resx	Adds CA2028 title/message/description/fix strings.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.zh-Hant.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.zh-Hans.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.tr.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.ru.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.pt-BR.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.pl.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.ko.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.ja.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.it.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.fr.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.es.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.de.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/Microsoft.NetCore.Analyzers/xlf/MicrosoftNetCoreAnalyzersResources.cs.xlf	Adds new CA2028 localized resource entries (state=new).
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers/AnalyzerReleases.Unshipped.md	Registers CA2028 in the unshipped analyzer release list.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers.sarif	Adds CA2028 rule metadata for SARIF.
src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers.md	Documents CA2028 in the analyzer rules list.

Copilot's findings

• Files reviewed: 20/20 changed files
• Comments generated: 4

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (4)

src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.CSharp.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/CSharpAvoidRedundantRegexIsMatchBeforeMatch.Fixer.cs:18

• using Microsoft.CodeAnalysis.Operations; is not used in this file. Please remove it to avoid unused using warnings.

using Microsoft.CodeAnalysis.Operations;
using Microsoft.NetCore.Analyzers;

src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.CSharp.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/CSharpAvoidRedundantRegexIsMatchBeforeMatch.Fixer.cs:199

• The fix currently strips trailing trivia from the original if condition (WithTrailingTrivia(TriviaList())). This can drop end-of-condition comments/formatting. Preserve the original condition trivia (or use WithTriviaFrom) so the code fix doesn't delete user comments.

            var newCondition = SyntaxFactory.IsPatternExpression(
                matchCallExpression.WithoutTrivia(),
                successPattern)
                .WithLeadingTrivia(ifStatement.Condition.GetLeadingTrivia())
                .WithTrailingTrivia(SyntaxFactory.TriviaList());

src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.CSharp.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/CSharpAvoidRedundantRegexIsMatchBeforeMatch.Fixer.cs:196

• matchCallExpression.WithoutTrivia() will drop any leading/trailing trivia (including comments) attached to the Regex.Match(...) initializer when it’s moved into the if condition. Please preserve trivia from the original initializer where possible.

            var newCondition = SyntaxFactory.IsPatternExpression(
                matchCallExpression.WithoutTrivia(),
                successPattern)

src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.CSharp.NetAnalyzers/Microsoft.NetCore.Analyzers/Runtime/CSharpAvoidRedundantRegexIsMatchBeforeMatch.Fixer.cs:205

• editor.RemoveNode(matchDeclarationStatement) uses the default remove options, which can drop leading/trailing trivia (e.g., comments) attached to the declaration statement. Consider using remove options that preserve trivia so the fixer doesn’t delete comments inside the if body.

            // Remove the Match declaration statement from the if body
            editor.RemoveNode(matchDeclarationStatement);

• Files reviewed: 21/21 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 1

[danmoseley]

@Youssef1313 good to merge?

[Youssef1313]

Best to get a review from @dotnet/dotnet-analyzers

[danmoseley]

@dotnet/dotnet-analyzers can you please review?