Using regex capture group in nginx $args

Question

As some background, I'm trying to fix requests that are coming in with duplicated params e.g:

/products/compare/?ids=554?ids=554
/products/compare/?ids=595,662,726?ids=595,662,726

My fix - which works - looks like this:

location /products/compare/ {
    if ( $args ~ "(ids=[\d,]+)\?ids=[\d,]+" ) {
        set $id $1;
        rewrite ^.*$ $scheme://$host/$uri$is_args$id? permanent;
    }
}

My question is regarding the set $id $1;, and why it's necessary

I was using the $1 capture-group variable directly in the rewrite:

rewrite ^.*$ $scheme://$host/$uri$is_args$1? permanent;

But the variable wasn't being populated.

Why not?

Just to clarify, when you use the set command, the variable does populate? btw, nice name :) — Will Barnwell
– Will Barnwell, Commented Jul 17, 2018 at 17:34

Richard Smith · Accepted Answer · 2018-07-17 19:03:32Z

6

The first parameter of the rewrite directive is a regular expression which may contain numbered captures. As a result, all numbered captures are reset when the rewrite statement is entered.

As an alternative to using the set directive, you could use a named capture within your if statement's regular expression.

For example:

if ( $args ~ "(?<id>ids=[\d,]+)\?ids=[\d,]+" ) {
    rewrite ^ $scheme://$host/$uri$is_args$id? permanent;
}

Of course, you do not actually need to use rewrite. If you use return instead, the numeric capture remains in scope.

For example:

if ( $args ~ "(ids=[\d,]+)\?ids=[\d,]+" ) {
    return 301 $scheme://$host/$uri$is_args$id;
}

answered Jul 17, 2018 at 19:03

Richard Smith

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1 Comment

Wintermute Over a year ago

Many thanks. And yes, good point about return rather than rewrite.

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