On: Regular Expression Engine in 14 lines of Python

[127]

Here's a complete PEG parser in 3 lines:

    consume = lambda c: lambda inp: inp[1:] if inp and inp[0] in c else None
    ordered_choice = lambda va,vb: lambda inp: va(inp) or vb(inp)
    concatenate = lambda va,vb: lambda inp: (lambda x: x and vb(x))(va(inp))

Used like this (something I hacked together recently): http://pastebin.com/dUyTZttZ

If you want it to actually do something useful, just wrap the functions with another functions that do what you need.

[abecedarius → 127]

In va(inp) or vb(inp) couldn't va(inp) succeed with an empty string as the result (at the end) which Python interprets as false, making it mistakenly try vb(x)? I haven't tried to run it.

Also I think c seems misleading as a variable name -- it suggests a single character, but the 'in c' means it's a set of characters.

[127 → abecedarius]

Yes, that is a bug and yes, those variable names do suck. Thanks for the correction.

    ordered_choice = lambda va,vb: lambda inp: (lambda a,b: a if a or a=="" else b)(va(inp),vb(inp))

Would work, but starts to be so ugly that I'd have to write proper functions and the code would not be 3 lines anymore. ;)

[abecedarius → 127]

How about

    eat = lambda chars: lambda s: s[1:] if s and s[0] in chars else None
    alt = lambda p, q: lambda s: (lambda r: q(s) if r is None else r)(p(s))
    seq = lambda p, q: lambda s: (lambda r: None if r is None else q(r))(p(s))

going a bit far in the short-names direction, oh well. :) It's too bad Python has this artificial statement/expression distinction.