Monks, I was looking at Synopsis 5, which is the intro to perl 6 regexes, or as they are now called, "rules." We have here 

# The special named assertions include:

    / <before pattern> /    # was /(?=pattern)/
    / <after pattern> /     # was /(?<pattern)/ 

    / <ws> /                # match whitespace by :w rules

    / <sp> /                # match a space char

The after assertion implements lookbehind by reversing the syntax tree
+ and looking for things in the opposite order going to the left. It i
+s illegal to do lookbehind on a pattern that cannot be reversed.

[download]

What does it mean that a pattern cannot be reversed?

Does this have anything to do with the unfortunate situation that variable length lookbehinds are not allowed in Perl 5? Will variable length lookbehinds be allowed in Perl 6? (Though I'm not sure this is even relevant to the concept of pattern reversability, just a guess...)

Googling on this concept was unhelpful.

Thanks for your wisdom!

thomas.

In reply to What does it mean that a "pattern cannot be reversed?" by tphyahoo

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