Subrule metacharacters

• Named subrules capture to hash elements of $/

# capture to $/<ident> and $/<expr>
regex assign { <ident> '=' <expr> }  

• An alias allows us to capture to a different name

# capture to $/<name> and $/<value>
regex assign { <name=ident> '=' <value=expr> } }

• A leading dot indicates a non-capturing subrule

regex ident { <.alpha> \w* }    # <alpha> not captured

• A leading ! is a zero-width negative match

regex { 'abc' <!digit> }        # abc not followed by a digit

• A leading ? is a zero-width positive match

regex { <?sigil> <variable> }   # parse variable only if sigil found