use strict;
my $str = 'p is prime. $`p_i`$ is the $`i^{th}`$ prime.
$`F(p)`$ is the set of factors `f` used to generate multiples of the prime `p`.
$`
\\begin{aligned}
\\dfrac{5}{2} r \\cdot \\dfrac{\\>\\>\\>\\frac{3}{2} w} {\\>1r}
&= \\frac{15}{4} w \\\\
&= \\>\\>3 \\frac{3}{4} \\>cups\\>water.
\\end{aligned}
`$
$`xyz`$
$``$
';
my $regex = qr/(\$`)(.*?)(`\$)/smp;
my $subst = '\\$$2\\$';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html