Are Java and C# regular expressions compatible?

Question

Both languages claim to use Perl style regular expressions. If I have one language test a regular expression for validity, will it work in the other? Where do the regular expression syntaxes differ?

The use case here is a C# (.NET) UI talking to an eventual Java back end implementation that will use the regex to match data.

Note that I only need to worry about matching, not about extracting portions of the matched data.

Answer 1

c# regex has its own convention for named groups (?<name>). I don't know of any other differences.

Answer 2

I find RegexBuddy invaluable when dealing with regex on multiple systems. Not an answer specifically, but you can convert between flavors easily and see the differences yourself.

Answer 3

.NET Regex supports counting, so you can match nested parentheses which is something you normally cannot do with a regular expression. According to Mastering Regular Expressions that's one of the few implementations to do that, so that could be a difference.

Answer 4

Java uses standard Perl type regex as well as POSIX regex. Looking at the C# documentation on regexs, it looks like that Java has all of C# regex syntax, but not the other way around.

Compare them yourself: Java: C#:

EDIT: Currently, no other regex flavor supports Microsoft's version of named capture.

Answer 5

Check out: http://www.regular-expressions.info/refflavors.html Plenty of regex info on that site, and there's a nice chart that details the differences between java & .net.

Answer 6

Differences are (from this site):

1. \Q...\E escapes a string of metacharacters
   • .NET NO
   • Java YES
2. \Q...\E escapes a string of character class metacharacters (in a character sets)
   • .NET NO
   • Java YES
3. (?n) (explicit capture modifier)
   • .NET YES
   • Java NO
4. ?+, *+, ++ and {m,n}+ (possessive quantifiers)
   • .NET NO
   • Java YES
5. (?<=text) (positive lookbehind)
   • .NET Full regex
   • Java Finite length
6. (?<!text) (negative lookbehind)
   • .NET Full regex
   • Java Finite length
7. Conditionals of form (?(?=regex)then|else), (?(regex)then|else), (?(1)then|else) or (?(group)then|else)
   • .NET YES
   • Java NO
8. (?#comment) comments
   • .NET YES
   • Java NO
9. Character class is a single token (Free-spacing syntax)
   • .NET YES
   • Java NO
10. \pL through \pC or \p{IsL} through \p{IsC} (Unicode properties)
    • .NET NO
    • Java YES
11. \p{IsLu} through \p{IsCn} (Unicode property)
    • .NET NO
    • Java YES
12. \p{InBasicLatin} through \p{InSpecials} or \p{IsBasicLatin} through \p{IsSpecials} (Unicode block)
    • .NET YES
    • Java NO
13. Spaces, hyphens and underscores allowed in all long names listed above (e.g. BasicLatin can be written as Basic-Latin or Basic_Latin or Basic Latin)
    • .NET NO
    • Java YES (Java 5)
14. Named captures of style (?<name>regex), (?'name'regex), \k<name> or \k'name'
    • .NET YES
    • Java NO
15. Multiple capturing groups can have the same name
    • .NET YES
    • Java N/A (does not have named capture groups)
16. XML character classes subtraction [abc-[abc]]
    • .NET YES (2.0)
    • Java NO
17. \p{Alpha} POSIX character class
    • .NET NO
    • Java YES (ASCII)