Using multiple Perl regular expressions to find and replace - regex

Question

I'm a Perl and regex newcomer in need of your expertise.
I need to process text files that include placeholder lines like Foo Bar1.jpg and replace those with with corresponding URLs like https:/baz/qux/Foo_Bar1.jpg.
As you may have guessed, I'm working with HTML. The placeholder text refers to the filename, which is the only thing available when writing the document. That's why I have to use placeholder text. Ultimately, of course, I want to replace the filename with the URL (after I upload file to my CMS to get the URL). At that point, I have all of the information at hand — the filename and the URL. Of course, I could just paste the URLs over the placeholder names in the HTML document. In fact, I've done that. But I'm certain that there's a better way.
In short, I have placeholder lines like this:
Foo Bar1.jpg
Foo Bar2.jpg
Foo Bar3.jpg
And I also have URL lines like this:
https:/baz/qux/Foo_Bar1.jpg
https:/baz/qux/Foo_Bar2.jpg
https:/baz/qux/Foo_Bar3.jpg
I want to find the placeholder string and capture a differentiator like Bar1 with a regex. Then I want to use the captured part like Bar1 to perform another regex search that matches part of the corresponding URL string, i.e. https:/baz/qux/Foo_Bar1.jpg. After a successful match, I want to replace the Foo Bar1.jpg line with https:/baz/qux/Foo_Bar1.jpg.
Ultimately, I want to do that for every permutation, so that https:/baz/qux/Foo_Bar2.jpg also replaces Foo Bar2.jpg and so on.
I've written regular expressions that match both the placeholder and the URL. That's not my problem, as far as I can tell. I can find the strings I need to process. For example, /[a-z]+\s([a-z0-9]+)\.jpg/ successfully matches what I'm calling the placeholder text and captures what I'm calling the differentiator.
However, though I've spent an embarrassing number of hours over the past week reading through Stack Overflow, various other sites and O'Reilly books on Pearl and Pearl Regular Expressions, I can't wrap my mind around how to process what I can find.

Answer 1

I think the piece you are missing is the idea of using Perl's internal grep function, for searching a list of URL lines based on what you are calling your "differentiator".
Slurp your URL lines into a Perl array (assuming there are a finite manageable number of them, so that memory is not clobbered):
open URLS, theUrlFile.txt or die "Cannot open.\n";
my #urls = <URLS>;
Then within the loop over your file containing "placeholders":
while (my $key = /[a-z]+\s([a-z0-9]+)\.jpg/g) {
my #matches = grep $key, #urls;
if (#matches) {
s/[a-z]+\s$key\.jpg/$matches[0]/;
}
}
You may also want to insert error/warning messages if #matches != 1.