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    <p>Ragel works fine. You just need to be careful about what you're matching. Your question uses both <code>[[tag]]</code> and <code>{{tag}}</code>, but your example uses <code>[[tag]]</code>, so I figure that's what you're trying to treat as special.</p> <p>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.</p> <p>Well, that's a verbatim description of this machine:</p> <pre><code>tag = '[[' lower+ ']]'; main := ( (any - '[')* # eat text ('[' ^'[' | tag) # try to eat a tag )*; </code></pre> <p>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:</p> <pre><code>static char *text_start; %%{ machine parser; action MarkStart { text_start = fpc; } action PrintTextNode { int text_len = fpc - text_start; if (text_len &gt; 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+ &gt;MarkStart) ']]' @PrintTagNode; main := ( (any - '[')* &gt;MarkStart %PrintTextNode ('[' ^'[' %PrintTextNode | tag) &gt;MarkStart )* @eof(PrintTextNode); }%% </code></pre> <p>There are a few non-obvious things:</p> <ul> <li>The <code>eof</code> action is needed because <code>%PrintTextNode</code> 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, <code>PrintTextNode</code> tests that it has some text to print.</li> <li>The <code>%PrintTextNode</code> action nestled in after the <code>^'['</code> is needed because, though we marked the start when we hit the <code>[</code>, after we hit a non-<code>[</code>, 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.</li> </ul> <p>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:</p> <pre><code>/* ragel so_tag.rl &amp;&amp; gcc so_tag.c -o so_tag */ #include &lt;stdio.h&gt; #include &lt;string.h&gt; static char *text_start; %%{ machine parser; action MarkStart { text_start = fpc; } action PrintTextNode { int text_len = fpc - text_start; if (text_len &gt; 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+ &gt;MarkStart) ']]' @PrintTagNode; main := ( (any - '[')* &gt;MarkStart %PrintTextNode ('[' ^'[' %PrintTextNode | tag) &gt;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 &lt; sizeof(buffer) &amp;&amp; feof(stdin)) eof = pe; %% write exec; if (eof || cs == %%{ write error; }%%) break; } while (1); return 0; } </code></pre> <p>Here's some test input:</p> <pre><code>[[header]] &lt;html&gt; &lt;head&gt;&lt;title&gt;title&lt;/title&gt;&lt;/head&gt; &lt;body&gt; &lt;h1&gt;[[headertext]]&lt;/h1&gt; &lt;p&gt;I am feeling very [[emotion]].&lt;/p&gt; &lt;p&gt;I like brackets: [ is cool. ] is cool. [] are cool. But [[tag]] is special.&lt;/p&gt; &lt;/body&gt; &lt;/html&gt; [[footer]] </code></pre> <p>And here's the output from the parser:</p> <pre><code>TAG(header) TEXT( &lt;html&gt; &lt;head&gt;&lt;title&gt;title&lt;/title&gt;&lt;/head&gt; &lt;body&gt; &lt;h1&gt;) TAG(headertext) TEXT(&lt;/h1&gt; &lt;p&gt;I am feeling very ) TAG(emotion) TEXT(.&lt;/p&gt; &lt;p&gt;I like brackets: ) TEXT([ ) TEXT(is cool. ] is cool. ) TEXT([]) TEXT( are cool. But ) TAG(tag) TEXT( is special.&lt;/p&gt; &lt;/body&gt; &lt;/html&gt; ) TAG(footer) TEXT( ) </code></pre> <p>The final text node contains only the newline at the end of the file.</p>
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