How to parse template languages in Ragel? - parsing

Question

I've been working on a parser for simple template language. I'm using Ragel.
The requirements are modest. I'm trying to find [[tags]] that can be embedded anywhere in the input string.
I'm trying to parse a simple template language, something that can have tags such as {{foo}} embedded within HTML. I tried several approaches to parse this but had to resort to using a Ragel scanner and use the inefficient approach of only matching a single character as a "catch all". I feel this is the wrong way to go about this. I'm essentially abusing the longest-match bias of the scanner to implement my default rule ( it can only be 1 char long, so it should always be the last resort ).
%%{
machine parser;
action start { tokstart = p; }
action on_tag { results << [:tag, data[tokstart..p]] }
action on_static { results << [:static, data[p..p]] }
tag = ('[[' lower+ ']]') >start #on_tag;
main := |*
tag;
any => on_static;
*|;
}%%
( actions written in ruby, but should be easy to understand ).
How would you go about writing a parser for such a simple language? Is Ragel maybe not the right tool? It seems you have to fight Ragel tooth and nails if the syntax is unpredictable such as this.

Answer 1

Ragel works fine. You just need to be careful about what you're matching. Your question uses both [[tag]] and {{tag}}, but your example uses [[tag]], so I figure that's what you're trying to treat as special.
What you want to do is eat text until you hit an open-bracket. If that bracket is followed by another bracket, then it's time to start eating lowercase characters till you hit a close-bracket. Since the text in the tag cannot include any bracket, you know that the only non-error character that can follow that close-bracket is another close-bracket. At that point, you're back where you started.
Well, that's a verbatim description of this machine:
tag = '[[' lower+ ']]';
main := (
(any - '[')* # eat text
('[' ^'[' | tag) # try to eat a tag
)*;
The tricky part is, where do you call your actions? I don't claim to have the best answer to that, but here's what I came up with:
static char *text_start;
%%{
machine parser;
action MarkStart { text_start = fpc; }
action PrintTextNode {
int text_len = fpc - text_start;
if (text_len > 0) {
printf("TEXT(%.*s)\n", text_len, text_start);
}
}
action PrintTagNode {
int text_len = fpc - text_start - 1; /* drop closing bracket */
printf("TAG(%.*s)\n", text_len, text_start);
}
tag = '[[' (lower+ >MarkStart) ']]' #PrintTagNode;
main := (
(any - '[')* >MarkStart %PrintTextNode
('[' ^'[' %PrintTextNode | tag) >MarkStart
)* #eof(PrintTextNode);
}%%
There are a few non-obvious things:
The eof action is needed because %PrintTextNode is only ever invoked on leaving a machine. If the input ends with normal text, there will be no input to make it leave that state. Because it will also be called when the input ends with a tag, and there is no final, unprinted text node, PrintTextNode tests that it has some text to print.
The %PrintTextNode action nestled in after the ^'[' is needed because, though we marked the start when we hit the [, after we hit a non-[, we'll start trying to parse anything again and remark the start point. We need to flush those two characters before that happens, hence that action invocation.
The full parser follows. I did it in C because that's what I know, but you should be able to turn it into whatever language you need pretty readily:
/* ragel so_tag.rl && gcc so_tag.c -o so_tag */
#include <stdio.h>
#include <string.h>
static char *text_start;
%%{
machine parser;
action MarkStart { text_start = fpc; }
action PrintTextNode {
int text_len = fpc - text_start;
if (text_len > 0) {
printf("TEXT(%.*s)\n", text_len, text_start);
}
}
action PrintTagNode {
int text_len = fpc - text_start - 1; /* drop closing bracket */
printf("TAG(%.*s)\n", text_len, text_start);
}
tag = '[[' (lower+ >MarkStart) ']]' #PrintTagNode;
main := (
(any - '[')* >MarkStart %PrintTextNode
('[' ^'[' %PrintTextNode | tag) >MarkStart
)* #eof(PrintTextNode);
}%%
%% write data;
int
main(void) {
char buffer[4096];
int cs;
char *p = NULL;
char *pe = NULL;
char *eof = NULL;
%% write init;
do {
size_t nread = fread(buffer, 1, sizeof(buffer), stdin);
p = buffer;
pe = p + nread;
if (nread < sizeof(buffer) && feof(stdin)) eof = pe;
%% write exec;
if (eof || cs == %%{ write error; }%%) break;
} while (1);
return 0;
}
Here's some test input:
[[header]]
<html>
<head><title>title</title></head>
<body>
<h1>[[headertext]]</h1>
<p>I am feeling very [[emotion]].</p>
<p>I like brackets: [ is cool. ] is cool. [] are cool. But [[tag]] is special.</p>
</body>
</html>
[[footer]]
And here's the output from the parser:
TAG(header)
TEXT(
<html>
<head><title>title</title></head>
<body>
<h1>)
TAG(headertext)
TEXT(</h1>
<p>I am feeling very )
TAG(emotion)
TEXT(.</p>
<p>I like brackets: )
TEXT([ )
TEXT(is cool. ] is cool. )
TEXT([])
TEXT( are cool. But )
TAG(tag)
TEXT( is special.</p>
</body>
</html>
)
TAG(footer)
TEXT(
)
The final text node contains only the newline at the end of the file.