Find matching bracket with Regex

Question

The input is a string representing a list of elements.

A list is defined as an open curly { followed by 0 or more elements separated by whitespace followed by a closed curly }.

An element is either a literal or a list of elements.

A literal is a succession of non-whitespace characters. If an element contains a curly bracket, it must be escaped with a backslash : \{ and \}. (Or you could assume curlies are not allowed inside literals, for simplicity)

Example:

"{abc { def ghi } 7 { 1 {2} {3 4} } {5 6} x\{yz \}foo }"

No curlies inside literals:

"{abc { def ghi } 7 { 1 {2} {3 4} } {5 6} xyz foo }"

(This is a simplified definition of a Tcl list.)

What I want to know is: can the input be split into the elements of the outermost loop using regex?

Expected output:

abc
{ def ghi }
7
{ 1 {2} {3 4} }
{5 6}
x{yz
}foo

The real question is: can this be done with a Regex?

I'm most interested in the .NET flavour, but will accept any answers.

I'll post my own assumption in an answer, and see if it's validated or destroyed.

Answer 1

Unfortunately the answer is YES for some flavor of Regex, e.g. PCRE and .NET because they support recursive pattern and stack-like operations respectively.

The grammar can be written as

ELEMENT  -> (?!\{)\S+ | LIST
LIST     -> '\{\s*' ELEMENT? ('\s+' ELEMENT)* '\s*\}' 

thus in PCRE, this can be transformed into the pattern:

   \{\s*(?0)?(?:\s+(?0))*\s*\}|(?!\{)(?:[^\s}]|\}(?![\s}]))+

#  ---------------------------                   ^^^^^^^^^
#            LIST                    Make sure the } is not closing the group

See http://www.ideone.com/SnGsU for example (I have stripped the top-level { and } for simplicity).

(Of course, don't try this at work :) )

(BTW, I don't know how to transform this PCRE into .NET flavor. If someone knows, please try Converting PCRE recursive regex pattern to .NET balancing groups definition)