Regex Patterns

Choose the Right Regex Engine Mode

🟡 DO use [GeneratedRegex] for all static regex patterns, but never remove NonBacktracking if present | .NET 7+

❌

var r = new Regex(dynamicPattern, RegexOptions.Compiled);

✅

[GeneratedRegex("pattern")]
private static partial Regex MyRegex();

var safe = new Regex(untrustedPattern, RegexOptions.NonBacktracking);

var oneOff = new Regex("pattern");

Impact: Source generator is always beneficial for static patterns. NonBacktracking prevents O(2^N) worst case — never remove it if present.

Use IsMatch When You Only Need a Boolean Result

🟡 DO use IsMatch instead of Match(...).Success | .NET 7+

❌

bool found = Regex.Match(input, pattern).Success;

✅

bool found = Regex.IsMatch(input, pattern);

Impact: Avoids Match object allocation; with NonBacktracking, ~3x faster by skipping capture computation.

Use Regex.Count/EnumerateMatches Instead of Matches

🟡 DO use Count() and EnumerateMatches() for allocation-free match processing | .NET 7+

❌

int count = 0;
Match m = regex.Match(text);
while (m.Success) { count++; m = m.NextMatch(); }

✅

int count = regex.Count(text);

foreach (ValueMatch m in Regex.EnumerateMatches(text, @"\b\w+\b"))
{
    ReadOnlySpan<char> word = text.AsSpan(m.Index, m.Length);
}

Impact: ~3x faster than Match/NextMatch with NonBacktracking. Zero allocations for both Count and EnumerateMatches.

Use Span-Based Regex APIs for Allocation-Free Matching

🟡 DO use ReadOnlySpan<char> overloads for regex matching on spans | .NET 7+

❌

string sub = largeBuffer.Substring(start, length);
bool found = Regex.IsMatch(sub, pattern);

✅

ReadOnlySpan<char> text = largeBuffer.AsSpan(start, length);
foreach (ValueMatch m in Regex.EnumerateMatches(text, @"\b\w+\b"))
{
    ReadOnlySpan<char> word = text.Slice(m.Index, m.Length);
}

Impact: Eliminates string allocations when working with spans — particularly valuable in high-throughput parsing pipelines.

Detection

Scan recipes for regex anti-patterns. Run these and report exact counts.

# Compiled regex count (startup cost budget — compare ratio to GeneratedRegex)
grep -rn --include='*.cs' 'RegexOptions.Compiled' --exclude-dir=bin --exclude-dir=obj . | wc -l

# GeneratedRegex count (already optimized — verify the inverse)
grep -rn --include='*.cs' 'GeneratedRegex' --exclude-dir=bin --exclude-dir=obj . | wc -l

# Uncached new Regex() calls (construction cost per call)
grep -rn --include='*.cs' 'new Regex(' --exclude-dir=bin --exclude-dir=obj . | wc -l

When RegexOptions.Compiled appears inside a class constructor or field initializer of an instantiated class (not a static singleton), count how many instances of that class are created at startup to determine total compiled regex budget. For example, if a Rule class compiles a regex in its constructor and 122 rules are registered, that is 122 compiled regexes at startup.

Patterns Requiring Manual Review

new Regex( uncached: Field assignment may span multiple lines — grep on one line is unreliable. Verify that matched instances are stored in static readonly fields or [GeneratedRegex].

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