Comment by silentbicycle | Hacker News Reader

Comment by silentbicycle | Hacker News Reader

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Regular expressions are for lexing (breaking up input into tokens, AKA "tokenizing"), parsers are for assembling a stream of tokens into a parse tree (according to a grammar). You can kinda-sorta get away with parsing via regular expressions, but "surprisingly complex" regular expressions (http://www.ex-parrot.com/~pdw/Mail-RFC822-Address.html) should be a sign that you're using the wrong tool. De-coupling lexing from parsing also means that your grammar won't be cluttered by issues like whitespace handling.

Perl's RE extensions can be used to do some parsing, albeit at the cost of potentially exponential runtime (http://swtch.com/~rsc/regexp/regexp1.html * ), since they're NP-complete (http://perl.plover.com/NPC/NPC-3SAT.html). (Congrats, your parser is now a denial-of-service attack vector!)

* If you find this article interesting, you'll probably love _Parsing Techniques_.

> I'm going to upvote this on the general principle that I've seen a lot of programmers who don't quite know what parsing is, but reading a 310-page book is perhaps not the best way to familiarize them with it.

This book is a _reference_. For someone who wants to learn basic parsing techniques, I'd first direct them to something like Niklaus Wirth's _Compiler Construction_ (PDF: http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf), which has a quick and clear introduction to recursive descent parsing, but only briefly touches on other methods.

The Grune book is for when you need to understand the trade-offs between (say) different kinds of chart parsers, how parser generators work, classic performance optimizations in Earley parsers, or error handling strategies. It has quite a bit more depth to it, as well as an excellent bibliography. The second edition was updated in 2008, and adds a lot of new content.

Since you brought up lexing I'm going to ask a really noob-ish question: does this book (by Grune) deal with lexing at all? If it doesn't, is it because parsing is the more interesting/complicated of the two?

I've recently started getting into a few "traditional" compiler resources after reading a lot of Lisp centric compiler books (Lisp in Small Pieces... SICP, PAIP) where lexing and parsing are kind of handled by (read) so this is a whole new world for me.

Lexing is strictly simpler than parsing - lexing is done with state machines* , while many parsers can be viewed as state machines with stacks, AKA "pushdown automata" (http://en.wikipedia.org/wiki/Pushdown_automata). Lexers don't have stacks, which is why basic regular expressions can't do things like match arbitrarily nested parenthesis.

* Basically: make a byte-by-byte flowchart ("an int is a digit, followed by more digits, stop when it reaches a non-digit character and return the accumulated string as an int"), then convert that to code. Pretty simple. Graphviz (http://graphviz.org) makes pretty state machine diagrams.

Just knowing a little bit about automata / state machines will be enough to write a decent lexer. The Wirth PDF above explains it just a few pages (chapter 3). People often use tools like lex or ragel because writing them by hand can be rather tedious (being able to say e.g. "[+-]?[1-9][0-9]* { return LEXEME_INT(atoi(str)); }" rather than writing out the tests byte-by-byte), but they're not hard.

And yeah, the Lispers are able to sidestep parsing entirely.

That rule doesn't hold for some languages. For example, a Python lexer needs to remember a stack of indentation levels to know whether a given line's indentation should be tokenized as an INDENT, a DEDENT, or no token at all.

True. Strictly speaking, that means it isn't context-free in the usual sense (right?), but it's a practical extension.

Matt Might uses Python's indentation as a lexing/parsing example in his ongoing "Scripting Language Design and Implementation" course (http://matt.might.net/teaching/scripting-languages/spring-20...), which is worth following if you're reading this thread.

That first paragraph was eye opening all on its own. I actually think I've collected masses of links to compiler resources from one or more of your comments in the past, so thanks for this one too.

Writing a state machine such that state transitions map to instruction pointer changes can be a viable technique. For example, using Pascal - handy because of sets - your example could be recognized thusly, with cp: PChar, or ^Char:

    start := cp;
    if cp^ in ['+', '-'] then Inc(cp);
    if cp^ in ['1'..'9'] then Inc(cp) else goto no_match;
    while cp^ in ['0'..'9'] do Inc(cp);
    SetString(str, start, cp - start);

I don't think this is tedious at all. The various components of a regular expression have analogues in code: * and + become loops, ? and | if statements, character classes become set membership tests (which is nice in Pascal). For most token definitions used in practical programming languages, writing lexers for them by hand is fairly trivial; and when things get more complicated, the freedom of having hand-coded the lexer usually gives you more tools to solve those complications.

Having a lexer coded as a loop driven by state transition tables can be useful in other scenarios, however; for example, if you only receive a little bit of input at a time, and you want to continuously lex it until a whole token has formed. There, you want to be able to suspend tokenizing at any point, and resume it later. Without coroutines, continuations or threads, this is more awkward when hand-coding.

Yes, it deals with everything involved in "parsing" which covers all aspects of starting from a text document and converting it into a tree structure that can be used either for code generation or for manipulating the code.

Lexing isn't all that different than parsing anyway. Both involve the recognition of "tokens" based on a stream of data. In the case of a lexer, the "tokens" are usually the characters taken character by character. For parsing, the "tokens" are identifiers, values, operators, etc.

From Perl 5.10 you can extend the RE engine.

One example of this is Regexp::Grammars (https://metacpan.org/module/Regexp::Grammars) which implements a complete recursive descent parser.

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