gistfile1.6pl

grammar YAML {
    my $indent = -1;
    token TOP {
        ^ <yaml> $
    }

    token yaml {
        <map>
    }

    token map {
        <indent>
        <pair>+
        <undent>
    }

    token pair {
        <ondent> <key> <separator> <value>
    }

    token separator {
        ':' [\h+ | \h*<?before \n>] 
    }

    token key {
        <-[:]>+
    }

    token value {
        <map> | <string>
    }

    token string {
        \w+
    }

    token indent {
        [
        | ^
        | <?before \n '  ' ** {$indent+1}> 
        ]
        { $indent++ } 
    }

    token ondent {
        [
        | ^
        | \n '  ' ** {$indent}
        ]
    }

    token undent {
        [
        | \n* $
        | <!before \n '  ' ** {$indent}>
        ]
        { $indent-- }
    }
}

my $yaml = q[
a: b
c:
  d: e
  f: g
h: i
];

say YAML.parse($yaml);