将Perl正则表达式转换为.NET

Question

I have some useful regular expressions in Perl. Is there a simple way to translate them to .NET's dialect of regular expressions?

If not, is there a concise reference of differences?

Answer 1

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 \  , 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\u003c/code> (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 eg \\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?) [az-[dw]]
• Balancing Group (?<-name>…) . This could be simulated with code evaluation assertion (?{…}) followed by a (?&name) .

References:

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

Answer 2

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 3

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.