How can we match a^n b^n with Java regex?

Question

_{This is the second part of a series of educational regex articles. It shows how lookaheads and nested references can be used to match the non-regular}

Answer 1

As mentioned in the question — with .NET balancing group, the patterns of the type aⁿbⁿcⁿdⁿ…zⁿ can be matched easily as

^
  (?a)+
  (?b)+  (?(A)(?!))
  (?c)+  (?(B)(?!))
  ...
  (?z)+  (?(Y)(?!))
$

For example: http://www.ideone.com/usuOE

---

Edit:

There is also a PCRE pattern for the generalized language with recursive pattern, but a lookahead is needed. I don't think this is a direct translation of the above.

^
  (?=(a(?-1)?b))  a+
  (?=(b(?-1)?c))  b+
  ...
  (?=(x(?-1)?y))  x+
     (y(?-1)?z)
$

For example: http://www.ideone.com/9gUwF