# Class Pxp_reader.rewrite_system_id

    class rewrite_system_id : ?forward_unmatching_urls:bool -> (string * string) list -> resolver -> resolver

Rewrites the URL's according to the list of pairs. The left component is the pattern, the right component is the substitute. For example,

`new rewrite_system_id
[ "http://host/foo/", "file:///dir/" ]
r
`

rewrites all URLs beginning with `http://host/foo/` to `file:///dir/`, e.g. `http://host/foo/x` becomes `file:///dir/x`.

If the pattern ends with a slash (as in the example), a prefix match is performed, i.e. the whole directory hierarchy is rewritten. If the pattern does not end with a slash, an exact match is performed, i.e. only a single URL is rewritten.

The class normalizes URLs as `norm_system_id` does, before the match is tried.

By default, URLs that do not match any pattern are rejected (`Not_competent`).

The rewritten URL is only visible within the passed subresolver. If the opened entity accesses other entities by relative URLs, these will be resolved relative to the original URL as it was before rewriting it. This gives some protection against unwanted accesses. For example, if you map `http://host/contents` to `file:///data/contents`, it will not be possible to access files outside this directory, even if tricks are used like opening `../../etc/passwd` relative to `http://host/contents`. Of course, this protection works only if the resolver opening the file is a subresolver of `rewrite_system_id`.

Another application of this class is to use the identity as rewriting rule. This resolver

`new rewrite_system_id
[ "file:///data/", "file:///data/" ]
( new resolve_as_file() )
`

has the effect that only files under `/data` can be accessed, and other such as `/etc/passwd` cannot.

Option `forward_unmatching_urls`: If true, URLs that do not match any pattern are forwarded to the inner resolver. These URLs are not rewritten. Note that the mentioned access restrictions do not work anymore if this option is turned on.