How can I match irreducible fractions with regex?
For example, 23/25, 3/4, 5/2, 100/101, etc.
First of all, I have no idea about the gcd-algorithm realization in regex.
Update for all of you who's answering like "You are using the wrong tool":
Yeah, guys, I'm realizing what regex is normally used for. It's okay. But that this question is weird is kind of its whole point.
Updated 2: The idea is to find a regex that could be helpful in a situation like:
$> echo "1/2" | grep -P regex
1/2
$> echo "2/4" | grep -P regex
So, the regex should be only a string, without using any scripts and variables. Only regex.
Actually, I already know some regex which match reducible fractions written in the unary number system.
$> echo "11/1111" | grep -P '^1/1+$|(11+)+\1+/\1+$'
11/1111
So the thing is to convert from decimal to unary number system in regex, but I don't know how.
Since the poster requested a single regex that matches against strings like "36/270", but says it doesn’t matter how legible it is, that regex is:
my $reducible_rx = qr{^(\d+)/(\d+)$(?(?{(1x$1."/".1x$2)=~m{^(?|1+/(1)|(11+)\1*/\1+)$}})|^)};
But, if like me, you believe that an illegible regex is absolutely unacceptable, you will write that more legibly as:
my $reducible_rx = qr{
# first match a fraction:
^ ( \d+ ) / ( \d+ ) $
# now for the hard part:
(?(?{ ( 1 x $1 . "/" . 1 x $2 ) =~ m{
^
(?| 1+ / (1) # trivial case: GCD=1
| (11+) \1* / \1+ # find the GCD
)
$
}x
})
# more portable version of (*PASS)
| ^ # more portable version of (*FAIL)
)
}x;
You can improve maintainability by splitting out the version that matches the unary version from the one that matches the decimal version like this:
# this one assumes unary notation
my $unary_rx = qr{
^
(?| 1+ / (1)
| (11+) \1* / \1+
)
$
}x;
# this one assumes decimal notation and converts internally
my $decimal_rx = qr{
# first match a fraction:
^ ( \d+ ) / ( \d+ ) $
# now for the hard part:
(?(?{( 1 x $1 . "/" . 1 x $2 ) =~ $unary_rx})
# more portable version of (*PASS)
| ^ # more portable version of (*FAIL)
)
}x;
Isn’t that much easier by separating it into two named regexes? That would now make $reducible_rx the same as $decimal_rx, but the unary version is its own thing. That’s how I would do it, but the original poster wanted a single regex, so you’d have to interpolate the nested one for that as I first present above.
Either way, you can plug into the test harness below using:
if ($frac =~ $reducible_rx) {
cmp_ok($frac, "ne", reduce($i, $j), "$i/$j is $test");
} else {
cmp_ok($frac, "eq", reduce($i, $j), "$i/$j is $test");
}
And you will see that it is a correct regex that passes all tests, and does so moreover using a single regex, wherefore having now passed all requirements of the original question, I declare Qᴜᴏᴅ ᴇʀᴀᴛ ᴅᴇᴍᴏɴsᴛʀᴀɴᴅᴜᴍ: “Quit, enough done.” 😇
And you’re welcome.
The answer is to match the regex ^(?|1+/(1)|(11+)\1*/\1+)$ against the fraction once it has been converted from decimal to unary notation, at which point the greatest common factor will be found in $1 on a match; otherwise they are coprimes. If you are using Perl 5.14 or better, you can even do this in one step:
use 5.014;
my $reg = qr{^(?|1+/(1)|(11+)\1*/\1+)$};
my $frac = "36/270"; # for example
if ($frac =~ s/(\d+)/1 x $1/reg =~ /$reg/) {
say "$frac can be reduced by ", length $1;
} else {
say "$frac is irreducible";
}
Which will correctly report that:
36/270 can be reduced by 18
(And of course, reducing by 1 means there is no longer a denominator.)
If you wanted to have a bit of punning fun with your readers, you could even do it this way:
use 5.014;
my $regex = qr{^(?|1+/(1)|(11+)\1*/\1+)$};
my $frac = "36/270"; # for example
if ($frac =~ s/(\d+)/"1 x $1"/regex =~ /$regex/) {
say "$frac can be reduced by ", length $1;
} else {
say "$frac is irreducible";
}
Here is the code that demonstrates how to do this. Furthermore, it constructs a test suite that tests its algorithm using all (positive) numerators and denominators up to its argument, or 30 by default. To run it under a test harness, put it in a file named coprimes and do this:
$ perl -MTest::Harness -e 'runtests("coprimes")'
coprimes .. ok
All tests successful.
Files=1, Tests=900, 1 wallclock secs ( 0.13 usr 0.02 sys + 0.33 cusr 0.02 csys = 0.50 CPU)
Result: PASS
Here is an example of its output when run without the test harness:
$ perl coprimes 10
1..100
ok 1 - 1/1 is 1
ok 2 - 1/2 is 1/2
ok 3 - 1/3 is 1/3
ok 4 - 1/4 is 1/4
ok 5 - 1/5 is 1/5
ok 6 - 1/6 is 1/6
ok 7 - 1/7 is 1/7
ok 8 - 1/8 is 1/8
ok 9 - 1/9 is 1/9
ok 10 - 1/10 is 1/10
ok 11 - 2/1 is 2
ok 12 - 2/2 is 1
ok 13 - 2/3 is 2/3
ok 14 - 2/4 is 1/2
ok 15 - 2/5 is 2/5
ok 16 - 2/6 is 1/3
ok 17 - 2/7 is 2/7
ok 18 - 2/8 is 1/4
ok 19 - 2/9 is 2/9
ok 20 - 2/10 is 1/5
ok 21 - 3/1 is 3
ok 22 - 3/2 is 3/2
ok 23 - 3/3 is 1
ok 24 - 3/4 is 3/4
ok 25 - 3/5 is 3/5
ok 26 - 3/6 is 1/2
ok 27 - 3/7 is 3/7
ok 28 - 3/8 is 3/8
ok 29 - 3/9 is 1/3
ok 30 - 3/10 is 3/10
ok 31 - 4/1 is 4
ok 32 - 4/2 is 2
ok 33 - 4/3 is 4/3
ok 34 - 4/4 is 1
ok 35 - 4/5 is 4/5
ok 36 - 4/6 is 2/3
ok 37 - 4/7 is 4/7
ok 38 - 4/8 is 1/2
ok 39 - 4/9 is 4/9
ok 40 - 4/10 is 2/5
ok 41 - 5/1 is 5
ok 42 - 5/2 is 5/2
ok 43 - 5/3 is 5/3
ok 44 - 5/4 is 5/4
ok 45 - 5/5 is 1
ok 46 - 5/6 is 5/6
ok 47 - 5/7 is 5/7
ok 48 - 5/8 is 5/8
ok 49 - 5/9 is 5/9
ok 50 - 5/10 is 1/2
ok 51 - 6/1 is 6
ok 52 - 6/2 is 3
ok 53 - 6/3 is 2
ok 54 - 6/4 is 3/2
ok 55 - 6/5 is 6/5
ok 56 - 6/6 is 1
ok 57 - 6/7 is 6/7
ok 58 - 6/8 is 3/4
ok 59 - 6/9 is 2/3
ok 60 - 6/10 is 3/5
ok 61 - 7/1 is 7
ok 62 - 7/2 is 7/2
ok 63 - 7/3 is 7/3
ok 64 - 7/4 is 7/4
ok 65 - 7/5 is 7/5
ok 66 - 7/6 is 7/6
ok 67 - 7/7 is 1
ok 68 - 7/8 is 7/8
ok 69 - 7/9 is 7/9
ok 70 - 7/10 is 7/10
ok 71 - 8/1 is 8
ok 72 - 8/2 is 4
ok 73 - 8/3 is 8/3
ok 74 - 8/4 is 2
ok 75 - 8/5 is 8/5
ok 76 - 8/6 is 4/3
ok 77 - 8/7 is 8/7
ok 78 - 8/8 is 1
ok 79 - 8/9 is 8/9
ok 80 - 8/10 is 4/5
ok 81 - 9/1 is 9
ok 82 - 9/2 is 9/2
ok 83 - 9/3 is 3
ok 84 - 9/4 is 9/4
ok 85 - 9/5 is 9/5
ok 86 - 9/6 is 3/2
ok 87 - 9/7 is 9/7
ok 88 - 9/8 is 9/8
ok 89 - 9/9 is 1
ok 90 - 9/10 is 9/10
ok 91 - 10/1 is 10
ok 92 - 10/2 is 5
ok 93 - 10/3 is 10/3
ok 94 - 10/4 is 5/2
ok 95 - 10/5 is 2
ok 96 - 10/6 is 5/3
ok 97 - 10/7 is 10/7
ok 98 - 10/8 is 5/4
ok 99 - 10/9 is 10/9
ok 100 - 10/10 is 1
And here is the program:
#!/usr/bin/env perl
#
# coprimes - test suite to use unary coprimality algorithm
#
# Tom Christiansen <tchrist@perl.com>
# Sun Apr 17 12:18:19 MDT 2011
use strict;
use warnings;
my $DEFAULT = 2*3*5;
my $max = @ARGV ? shift : $DEFAULT;
use Test::More;
plan tests => $max ** 2;
my $rx = qr{
^
(?| 1+ / (1)
| (11+) \1* / \1+
)
$
}x;
for my $i ( 1 .. $max ) {
for my $j ( 1 .. $max ) {
my $test;
if (((1 x $i) . "/" . (1 x $j)) =~ /$rx/) {
my $cf = length($1);
$test = $i / $cf;
$test .= "/" . $j/$cf unless $j/$cf == 1;
} else {
$test = "$i/$j";
}
cmp_ok($test, "eq", reduce($i, $j), "$i/$j is $test");
}
}
sub reduce {
my ($a, $b) = @_;
use Math::BigRat;
my $f = new Math::BigRat "$a/$b";
return "$f";
}