Regular Expression Engine Performance Considerations

There are five techniques for coupling the regex engine to a regex pattern:

  1. Instantiate a RegEx object (interpreted regular expressions)
  2. Call a static pattern-matching method (interpreted)
  3. Instantiate a RegEx object with the Compiled option (compiled regular expressions)
  4. Create a RegEx object and save it to a standalone assembly (Regex.CompileToAssembly)
  5. Use the .NET regular expression source generator, RegexGenerator (requires .NET 7+)
TechniqueInvoked viaConstruction CostExecution CostConsiderationsUse case
Interpretednew Regex()
a Regex static method
LowHighCost is incurred each instantiationA Regex that is called infrequently and high performance cost is acceptable
CompiledRegexOptions.CompiledHighLowAdds to startup costA Regex that is called frequently in .NET 6 or earlier
AssemblyRegex.CompileToAssemblyLowLowDifficult to use
Difficult to debug
Low performance cost and smaller app size is required
Source generationGeneratedRegexAttributeLowLowSupports AOT compilation
Emits more source, increase app size
A Regex that is called frequently in .NET 7 or later

Avoid Repeated Regex Object Instantiation

Each time a Regex object is instantiated, the regular expression pattern passed to it must be recompiled. This is an expensive operation.

Below, each time IsValidCurrency is called, a new Regex object is instantiated:

public static bool IsValidCurrency(string currencyValue)
    string pattern = @"\p{Sc}+\s\d+";
    Regex currencyRegex = new Regex(pattern); // This object is instantiated and the regular expression pattern is compiled
    return currencyRegex.IsMatch(currencyValue);

Instead, consider this call to a static Regex method:

public class RegexLib
   public static bool IsValidCurrency(string currencyValue)
      string pattern = @"\p{Sc}+\s*\d+";

      return Regex.IsMatch(currencyValue, pattern);

Using Source Generated Regular Expressions (#5)

Availability: .NET 7

A source generator is a component that plugs into the compiler, is given information about the code like an analyzer, and can augment the compiled output with additional source code.

The RegexGenerator (source generator) is invoked on partial methods decorated with GeneratedRegexAttribute with a return type Regex.

[GeneratedRegex("abc|def", RegexOptions.IgnoreCase, "en-US")]
private static partial Regex AbcOrDefGeneratedRegex();

private static void EvaluateText(string text)
    if (AbcOrDefGeneratedRegex().IsMatch(text))
        // ...

The generated source code can be debugged like any other source code.

Other Performance Considerations


Ordinarily, the regex engine uses linear progression to traverse an input string and compare it to a regex pattern. Indeterminate quantifiers like *, +, and ? allow the regex engine to give up a partial successful match to search for a successful match in the entire pattern. This is backtracking.

Backtracking can significantly degrade performance. Worst case, execution time doubles for each additional character in input. If backtracking is not necessary, either:

  1. Disable it with an atomic group: (?>subexpression), or;
  2. Pass RegexOptions.NonBacktracking. This guarantees linear progression and avoids backtracking.

Nonbacktracking Mode Considerations

NonBacktracking is not compatible with:

  • RegexOptions.RightToLeft
  • RegexOptions.ECMASCript
  • Atomic groups
  • Backreferences
  • Balancing groups
  • Conditionals
  • Lookarounds
  • Start anchors (\G)

If a capture group is in a loop, NonBacktracking will only return the last matched value for that capture. This is different from the normal behavior of returning all matched values.


Always set a timeout to minimize effect of excessive backtracking, if it occurs.

Setting a timeout value:

  • The Regex type has an overloaded constructor that accepts a TimeSpan value.
  • The Regex.Match and Regex.Replace methods have an overload that include a matchTimeout parameter.
  • Set process-wide or AppDomain-wide timeouts:
    • AppDomain.CurrentDomain.SetData("REGEX_DEFAULT_MATCH_TIMEOUT", TimeSpan.FromMilliseconds(timeoutValueInMilliseconds));

If timeouts are reached, a RegexMatchTimeoutException is thrown.

Capturing Groups

Use capturing groups such as (subexpression) and (?<name>subexpression) only when necessary and disable otherwise. These constructs are expensive.

Options to disable:

  • Use (?:subexpression) which prevents the capture of matched substrings (does not disable substring captures in nested groups).
  • Use the ExplicitCapture option which disables all unnamed or implicit captures in the the regular expression pattern.