Wow, you guys are amazingly quick to reply and helpful in those replies. That said, since I need to do this outside of Perl, in a pure PCRE expression, neither this nor ikegami's post is helpful.

I disagree. We didn't use anything Perl-specific. For example, here it is in JScript:

var pat = "\\d\\s+foo";
var regexp = "(" + pat + ")[^\r\n]*(" + pat + ")";
var re = new RegExp(regexp);
re.exec(some_string);
[download]

(Untested)