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 ex

Answer 1

From my experience:

Java 7 regular expressions as compared to .NET 2.0 regular expressions:

• Underscore symbol in group names is not supported

• Groups with the same name (in the same regular expression) are not supported (although it may be really useful in expressions using "or"!)

• Groups having captured nothing have value of null and not of an empty string

• Group with index 0 also contains the whole match (same as in .NET) BUT is not included in groupCount()

• Group back reference in replace expressions is also denoted with dollar sign (e.g. $1), but if the same expression contains dollar sign as the end-of-line marker - then the back reference dollar should be escaped (\$), otherwise in Java we get the "illegal group reference" error

• End-of-line symbol ($) behaves greedy. Consider, for example, the following expression (Java-string is given): "bla(bla(?:$|\r\n))+)?$". Here the last line of text will be NOT captured! To capture it, we must substitute "$" with "\z".

• There is no "Explicit Capture" mode.

• Empty string doesn't satisfy the ^.{0}$ pattern.

• Symbol "-" must be escaped when used inside square brackets. That is, pattern "[a-z+-]+" doesn't match string "f+g-h" in Java, but it does in .NET. To match in Java, pattern should look as (Java-string is given): "[a-z+\-]+".

NOTE: "(Java-string is given)" - just to explain double escapes in the expression.