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

[krwq]

Couple of comments about potential false positives from AI to verify

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 2

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[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: 0 new

[danmoseley]

The failures are unrelated.

[krwq]

Per AI:

The diagnostic doesn't fire — but not because of the new IInstanceReferenceOperation restriction. It doesn't fire because the existing readonly-field
equivalence check already rejects non-readonly fields. To actually exercise the new behavior, the field must be public readonly Regex r;. As written, this test would pass even if the new restriction were reverted.

[krwq]

LGTM but one test case is probably always passing and need extra readonly on the field. Note that this is only partially reviewed manually, most of the feedback is from AI.

[danmoseley]

/ba-g unrelated