## Why is this an issue?

Regular expressions have their own syntax that is understood by regular expression engines. Those engines will throw an exception at runtime if they are given a regular expression that does not conform to that syntax.

To avoid syntax errors, special characters should be escaped with backslashes when they are intended to be matched literally and references to capturing groups should use the correctly spelled name or number of the group.

Negative lookaround groups cannot be combined with RegexOptions.NonBacktracking. Such combination would throw an exception during runtime.

## How to fix it

### Code examples

#### Noncompliant code example

    void Regexes(string input)
    {
        var regex = new Regex("[A");                                                    // Noncompliant
        var match = Regex.Match(input, "[A");                                           // Noncompliant
        var negativeLookahead = new Regex("a(?!b)", RegexOptions.NonBacktracking);      // Noncompliant
        var negativeLookbehind = new Regex("(?<!a)b", RegexOptions.NonBacktracking);    // Noncompliant
    }

#### Compliant solution

    void Regexes(string input)
    {
        var regex = new Regex("[A-Z]");
        var match = Regex.Match(input, "[A-Z]");
        var negativeLookahead = new Regex("a(?!b)");
        var negativeLookbehind = new Regex("(?<!a)b");
    }

## Resources

### Documentation

- .NET Regular expressions
- Lookarounds at a glance
- Backtracking in Regular Expressions
- Character Escapes in Regular Expressions