I've donea 2nd version of this <http://scsys.co.uk:8002/391796>, which I
think should be faster and, especially, take less memory.
This technique is, I hope, of wide interest. To do an "unanchored"
search, it uses Marpa's :discard mechanism. Essentially, it treats
strings that are not part of the search target as whitespace.
The SLIf is quite compact, but an explanation may help. I set the
grammar up to discard all single characters of length 1:
:discard ~ [\d\D]
Discard will always be the last choice. Even in LATM, longest match
wins and, with length 1, a discard lexeme can at worst tie for longest
match. Whenever there is a non-discard match, that is preferred.
Bottom line: discard always loses, unless there is no other choice.
Then, for your search patterns, you define other lexemes. As just
shown, when they match they will always be preferred. If you want all
matches, you make your top rule
string ::= target+
where <target> is the pattern you are searching for.
-- jeffrey
On 06/07/2014 10:48 PM, Steven Haryanto wrote:
Hi all,
I wonder if it's feasible to use Marpa, like regular expression, to
detect some pattern inside a string. An example of what I'm trying to
do is to extract some numeric expression from these strings:
"1+2"
"This is an expression: 1+2, and this is another 1+2+4"
"1+2 is the expression"
I want to recognize and extract 1+2 and 1+2+4 from the above. Here's
my current (and failing) attempt:
---
use MarpaX::Simple qw(gen_parser);
my $p = gen_parser(
grammar => <<'_',
lexeme default = latm => 1
:default ::= action=>::first
:start ::= answer
answer ::= expr
| expr any
| any expr action=>get1
| any expr any action=>get1
expr ::= num
| expr '+' expr
num ~ [\d]+
any ~ [\d\D]+
:discard ~ ws
ws ~ [\s]+
_
trace_terminals => 1,
trace_values => 1,
actions => {
get0 => sub { $_[1] },
get1 => sub { $_[2] },
},
);
sub check { say "Input: $_[0]"; say "Output: ", $p->($_[0]); say "=" x
20 }
check('1');
check('1 + 2');
check('1 + 2 is the expression');
check('This is an expression: 1 + 2 and another 1+2+4');
--
Regards,
Steven
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