What I’m Trying to Accomplish

1. I have more than 1,000 individual files in the format https://example.com/archives/<NNNNNN>.php where N is an integer. Here is an example.

2. For some very complicated historical reasons, I need to keep the existing structure as-is, though I can add to it. In this case, each page has a unique data-url-title="<url-friendly-title>".

3. I would like my .htaccess config to determine what file was read, use regex to extract the url-friendly-title and rewrite the final URL to replace <NNNNNN>, in the format https://example.com/archives/url-friendly-title.php (I would then strip .php from it—but I didn’t get that far).

What I Try to Do

# Enable rewriting
RewriteEngine On

# Match the old archive URL
RewriteCond %{REQUEST_URI} ^/archives/([0-9]+).php$

# Check that the file exists
RewriteCond expr "-f '%{DOCUMENT_ROOT}/archives/%1.php'"

# Extract data-url-title from the file content
RewriteCond expr "file('%{DOCUMENT_ROOT}/archives/%1.php') =~ /data-url-title="([^"]+)"/"

# Rewrite the URL to the new friendly URL
RewriteRule ^archives/([0-9]+).php$ /archives/%1.php [R=301,L]

What Happens

Any attempt to use this configuration causes my entire website to throw an internal server error with the log message RewriteCond: bad flag delimiters.

• I don’t think I have any spaces in my regex.

• I’ve converted my .htaccess file to Unix endings.

I have looked at numerous accepted answers, and they seem to indicate that, in Apache 2.4.x, it’s completely legitimate to read a variable from an external file by using RewriteCond expr "<expression>".

I can’t get this to work. Moreover, the htaccess tester throws Unsupported TestString: expr for both RewriteCond expr statements. What am I missing here?