string.replace not working

["Patrick Wiseman" <pwiseman@gmail.com>]

Hello:

A new repository of US caselaw[1] has recently been released, and I'm
trying to make it a little more useful.

At this URL (which happens to be Brown v. Bd. of Ed.),
http://bulk.resource.org/courts.gov/c/US/347/347.US.483.html, I'm
trying to convert data which looks so, " 123 U.S. 456", into a link,
so, "<a href=\"../123/123.US.456.html\"> 123 U.S. 456</a&gt".

This text.replace works on a literal string: "blah blah 123 U.S. 456
blah blah 789 U.S. 123".replace(/\s+(\d+)\s+U\.S\.\s+(\d+)/g, " <a
href=\"../$1/$1.US.$2.html\">$&</a>"); replacing both
citations with the appropriate link.

but I cannot get it to work on the document.  Here's the script:

var b = document.body.innerHTML;
b.replace(/\s+(\d+)\s+U\.S\.\s+(\d+)/g, " <a
href=\"../$1/$1.US.$2.html\">$&</a>");
b = "<!--modified content-->" + b;
document.body.innerHTML = b;

I stuck the 'modified content' comment in there as a sanity check, and
it appears in the modified source.  I tried wrapping the last line
inside a window.addEventListener load statement, but it made no
difference.  I have a feeling I'm missing something incredibly
obvious!  I re-read the common pitfalls chapter, but saw nothing
there.  Any insights would be much appreciated.

pw
[1] See http://bulk.resource.org/courts.gov/