The difference between top-down parsing and bottom-up parsing

I really didn&#x27;t understand grammars until I started doing hand-rolled top-down recursive descent parsing for Gosu.<p>It&#x27;s a shame that parsing in CS schools is taught using theory and lex&#x2F;yacc type tools when a basic lexer and recursive descent parser can be written from first principles in a few months. It is more incremental, and it gives you a much deeper feel for the concepts, plus it lets you learn a bit about software engineering and organization as well.

(facetious-comment &quot;It&#x27;s a tragedy how much brilliance is wasted on grammar parsing when it&#x27;s a solved problem; just use a Lisp&quot;)

Bottom up parsing - &quot;If not, it&#x27;s necessary to backtrack and try combining tokens in different ways&quot; I feel the way it is put along with shift reduce parsing is misleading. Backtracking is essentially an aspect avoided (more like solved) by shift-reduce parsing. They don&#x27;t go together in bottom up parsing. Shift reduce parsers posses the potential to predict the handle to use by looking at the contents on top of the stack.<p>Good job BTW, there are very few people who write about compiler&#x2F;language theory :)

One more difference: top-down parsing is European and bottom-up - American :)

I was so happy when I finally grokked LR parsers: it&#x27;s just a big state machine! _if_ that token found, push on stack, go to _that_ other state. Consume token, check the next state transition.<p>But while fun it was pretty much useless because recursive descents and combinators are so much easier.

Another good write-up on this topic is <a href="http://blog.reverberate.org/2013/07/ll-and-lr-parsing-demystified.html?m=1" rel="nofollow">http:&#x2F;&#x2F;blog.reverberate.org&#x2F;2013&#x2F;07&#x2F;ll-and-lr-parsing-demyst...</a>

I think I read somewhere incremental parsers are better off being written with bottomup parsers rather than topdown parsers. The reason was that when a small edit is made to the code being parsed, the artifact from a bottomup parser often only needs a minor change that ripples only as far as it needs to, whereas the topdown parser needs to be completely rerun because it can&#x27;t tell whether the effect of one small edit is large or localized. Anyone out there who can verify or refute this?

What do you call it if you&#x27;re just using a big list of regexes? I&#x27;ve seen that used for a simple dialog scripting language in a game.