translate Perl regex to .NET

Question

I have some useful regexes in Perl. Is there a simple way to translate them to .NET's dialect of regex? If not, is there a concise reference of differences?

Answer 1

They were designed to be compatible with Perl 5 regexes. As such, Perl 5 regexes should just work in .NET.

You can translate some RegexOptions as follows:

[Flags]
public enum RegexOptions
{
  Compiled = 8,
  CultureInvariant = 0x200,
  ECMAScript = 0x100,
  ExplicitCapture = 4,
  IgnoreCase = 1,                 // i in Perl
  IgnorePatternWhitespace = 0x20, // x in Perl
  Multiline = 2,                  // m in Perl
  None = 0,
  RightToLeft = 0x40,
  Singleline = 0x10               // s in Perl
}

Another tip is to use verbatim strings so that you don't need to escape all those escape characters in C#:

string badOnTheEyesRx    = "\\d{4}/\\d{2}/\\d{2}";
string easierOnTheEyesRx = @"\d{4}/\d{2}/\d{2}";

Answer 2

It really depends on the complexity of the regular expression - many ones will work the same out of the box.

Take a look at this .NET regex cheat sheet to see if an operator does what you expect it to do.

I don't know of any tool that automatically translates between RegEx dialects.

Answer 3

There is a big comparison table in http://www.regular-expressions.info/refflavors.html.

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Most of the basic elements are the same, the differences are:

Minor differences:

• Unicode escape sequences. In .NET it is \u200A, in Perl it is \x{200A}.
• \v in .NET is just the vertical tab (U+000B), in Perl it stands for the "vertical whitespace" class. Of course there is \V in Perl because of this.
• The conditional expression for named reference in .NET is (?(name)yes|no), but (?(<name>)yes|no) in Perl.

Some elements are Perl-only:

• Possessive quantifiers (x?+, x*+, x++ etc). Use non-backtracking subexpression ((?>…)) instead.
• Named unicode escape sequence \N{LATIN SMALL LETTER X}, \N{U+200A}.
• Case folding and escaping
  • \l (lower case next char), \u (upper case next char).
  • \L (lower case), \U (upper case), \Q (quote meta characters) until \E.
• Shorthand notation for Unicode property \pL and \PL. You have to include the braces in .NET e.g. \p{L}.
• Odd things like \X, \C.
• Special character classes like \v, \V, \h, \H, \N, \R
• Backreference to a specific or previous group \g1, \g{-1}. You can only use absolute group index in .NET.
• Named backreference \g{name}. Use \k<name> instead.
• POSIX character class [[:alpha:]].
• Branch-reset pattern (?|…)
• \K. Use look-behind ((?<=…)) instead.
• Code evaluation assertion (?{…}), post-poned subexpression (??{…}).
• Subexpression reference (recursive pattern) (?0), (?R), (?1), (?-1), (?+1), (?&name).
• Some conditional expression's predicate are Perl-specific:
  • code (?{…})
  • recursive (R), (R1), (R&name)
  • define (DEFINE).
• Special Backtracking Control Verbs (*VERB:ARG)
• Python syntax
  • (?P<name>…). Use (?<name>…) instead.
  • (?P=name). Use \k<name> instead.
  • (?P>name). No equivalent in .NET.

Some elements are .NET only:

• Variable length look-behind. In Perl, for positive look-behind, use \K instead.
• Arbitrary regular expression in conditional expression (?(pattern)yes|no).
• Character class subtraction (undocumented?) [a-z-[d-w]]
• Balancing Group (?<-name>…). This could be simulated with code evaluation assertion (?{…}) followed by a (?&name).

References:

• .NET Framework 4: Regular Expression Language Elements
• perlre