Hello monks,

After playing a bit with perl 5.10 regexps I have being able to work a primitive infix to postfix translator with them:

pl@nereida:~/Lperltesting$ cat ./calc510withactions2.pl
#!/usr/local/lib/perl/5.10.1/bin//perl5.10.1
use v5.10;

# Infix to postfix translator using 5.10 regexp
# Original grammar:

#  exp ->   exp [-+] term
#         | term
#  term ->   term [*/] digits
#          | digits

# Applying left-recursion elimination we have:

#  exp  ->   term re
#  re   ->   [+-] term re
#          | # empty
#  term ->   digits rt
#  rt   ->   [*/] rt
#          | # empty

my @stack;
my $regexp = qr{
    (?&exp)

    (?(DEFINE)
        (?<exp>    (?&term) (?&re)
        )

        (?<re>     \s* ([+-]) (?&term) \s* (?{ push @stack, $^N }) (?&
+re)
                 | # empty
        )

        (?<term>   (?&digits) (?&rt)
        )

        (?<rt>     \s*([*/]) (?&digits) \s* (?{ push @stack, $^N }) (?
+&rt)
                  |  # empty
        )

        (?<digits>  \s* (\d+)
           (?{ push @stack, $^N })
        )
    )
}xms;

my $input = <>;
chomp($input);
if ($input =~ $regexp) {
  say "matches: $&\nStack=(@stack)";
}
else {
  say "does not match";
}
[download]

Minus and divide operators seem to behave according the expected left associativity:

pl@nereida:~/Lperltesting$ ./calc510withactions2.pl
2-8*4/2/4-1
matches: 2-8*4/2/4-1
Stack=(2 8 4 * 2 / 4 / - 1 -)
[download]

I feel however that I can't extend this methodology to more complex tasks because I haven't found a way other than $^N to refer to the values associated with the previous parenthesis. Which means that I can only refer to the attribute of the last parenthesis.

Is in Perl 5.10 something like relative variable backreferences so that I can use something like $-1, $-2, etc. to refer to the last parenthesis, the one before the last, etc. inside the embeded code? I.e. I am looking for s.t. similar to relative backreferences like \g{-1} but to be used inside inserted code?

Apologies for not being able to clarify the question enough. I feel I do not have yet a clear understanding of Perl 5.10 regexps.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.