Re: Order of definitions in source-highlight 2.10

[email protected] wrote:
> I just upgraded source-highlight to 2.10 and I am noticing some strange 
> behavior.
> 
> Suppose we have the file foo.lang:
> 
> symbol = "/"
> comment start "//"
> 
> And the file test.foo:
> 
> // foo
> 
> The language definition is taken from the source-highlight manual, 
> section 7.4: "Order of definitions".  Note that the definitions are in 
> the wrong order, according to the manual: "The first expression will 
> always be matched first, and the second expression will never be 
> matched."  And yet:
> 
> $ source-highlight --lang-def=foo.lang -c foo.css --no-doc -i test.foo
> <!-- Generator: GNU source-highlight 2.10
> by Lorenzo Bettini
> http://www.lorenzobettini.it
> http://www.gnu.org/software/src-highlite -->
> <pre><tt><span class="comment">// foo</span>
> </tt></pre>
> 
> This was different with version 2.9:
> 
> $ source-highlight --lang-def=foo.lang -c foo.css --no-doc -i test.foo
> <!-- Generator: GNU source-highlight 2.9
> by Lorenzo Bettini
> http://www.lorenzobettini.it
> http://www.gnu.org/software/src-highlite -->
> <pre><tt><span class="symbol">//</span><span class="normal"> foo</span>
> </tt></pre>
> 
> What has changed between version 2.9 and 2.10?

Hi

yes, the strategy for regular expression matching has changed: before it 
used to build a huge regular expression with many alternatives; however, 
this would make the handling of things such as backreferences a real 
nightmare (since the number of backreference would have to be updated, 
and the number of backreferences is limited to 9), in particular it 
required to split regular expressions and the code was really buggy.

so in 2.10 I completely re-written the handling of regular expressions 
(http://www.gnu.org/software/src-highlite/source-highlight.html#fn-29); 
in particular, now each element has its own regular expression and the 
engine tests each expression and, as explained in 7.12:

"As hinted at the beginning of Language Definitions, source-highlight 
uses the definitions in the language definition file to internally 
create, on-the-fly, regular expressions that are used to highlight the 
tokens of an input file. Here we provide some internal details that are 
crucial to understand how to write language definition files correctly29.

First of all, each element definition, an highlighting rule is created 
by source-highlight (even if they correspond to the same language 
element); thus, each language definition file will correspond to a list 
of highlighting rules. For each line of the input file, source-highlight 
will try to match all these rules against the whole line (more formally, 
against the part of the line that has not been highlighted yet). It will 
not stop as soon as an highlighting rule matched, since there might be 
another rule that matches “better”.

The strategy used by source-highlight is to select the first rule that 
matches the longest part of the text with the smallest prefix (i.e., the 
initial part of the line that contains no language element). (Thus, as 
already noted in the previous sections, the order of language 
definitions is crucial.) Then, it will continue to search for another 
matching rule for the remaining part of the line."

So the case of / and // respects this rule, since // matches better than /.

Of course, you're right: the example of 7.4 does not work anymore and I 
have to update the documentation with a better example!  Sorry about 
that, and thanks for the bug report.

Does this new strategy pose problems for your language definition?

hope to hear from you soon
cheers
	Lorenzo

-- 
Lorenzo Bettini, PhD in Computer Science, DI, Univ. Torino
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