Apache Module mod_substitute

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Description:	Perform search and replace operations on response bodies
Status:	Extension
Module Identifier:	substitute_module
Source File:	mod_substitute.c
Compatibility:	Available in Apache HTTP Server 2.2.7 and later

Summary

mod_substitute provides a mechanism to perform both regular expression and fixed string substitutions on response bodies.

Directives

Substitute

Comments

Substitute Directive

Description:	Pattern to filter the response content
Syntax:	`Substitute s/pattern/substitution/[infq]`
Context:	directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_substitute

The Substitute directive specifies a search and replace pattern to apply to the response body.

The meaning of the pattern can be modified by using any combination of these flags:

i: Perform a case-insensitive match.
n: By default the pattern is treated as a regular expression. Using the n flag forces the pattern to be treated as a fixed string.
f: The f flag causes mod_substitute to flatten the result of a substitution allowing for later substitutions to take place on the boundary of this one. This is the default.
q: The q flag causes mod_substitute to not flatten the buckets after each substitution. This can result in much faster response and a decrease in memory utilization, but should only be used if there is no possibility that the result of one substitution will ever match a pattern or regex of a subsequent one.

Example

<Location />
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute s/foo/bar/ni
</Location>

If either the pattern or the substitution contain a slash character then an alternative delimiter should be used:

Example of using an alternate delimiter

<Location />
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s|<BR */?>|<br />|i"
</Location>

Backreferences can be used in the comparison and in the substitution, when regular expressions are used, as illustrated in the following example:

Example of using backreferences and captures

<Location />
    AddOutputFilterByType SUBSTITUTE text/html
    # "foo=k,bar=k" -> "foo/bar=k" 
    Substitute "s|foo=(\w+),bar=\1|foo/bar=$1"
</Location>

A common use scenario for mod_substitute is the situation in which a front-end server proxies requests to a back-end server which returns HTML with hard-coded embedded URLs that refer to the back-end server. These URLs don't work for the end-user, since the back-end server is unreachable.

In this case, mod_substutite can be used to rewrite those URLs into something that will work from the front end:

Rewriting URLs embedded in proxied content

ProxyPass /blog/ http://internal.blog.example.com
ProxyPassReverse /blog/ http://internal.blog.example.com/

Substitute "s|http://internal.blog.example.com/|http://www.example.com/blog/|i"

ProxyPassReverse modifies any Location (redirect) headers that are sent by the back-end server, and, in this example, Substitute takes care of the rest of the problem by fixing up the HTML response as well.