RegexOptions

private static Regex htmlRegex = new Regex(“<.*?>”, RegexOptions.Compiled);

is valid C# syntax, but Unity complains:

error CS0117: `System.Text.RegularExpressions.RegexOptions' does not contain a definition for `Compiled'
error CS1502: The best overloaded method match for `System.Text.RegularExpressions.Regex.Regex(string, System.Text.RegularExpressions.RegexOptions)' has some invalid arguments
error CS1503: Argument `#2' cannot convert `object' expression to type `System.Text.RegularExpressions.RegexOptions'

Why?

Edit → Project Settings → Player → Optimization

Api Compatibility Level => .NET 2.0