Apache HTTP Server Version 2.5

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# Apache Module mod_substitute

Description: | Perform search and replace operations on response bodies
Status: | Extension
Module Identifier: | substitute_module
Source File: | mod_substitute.c

### Summary

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

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

## 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
Compatibility: | "expr=" substitution values were added in 2.5.1

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.

The substitution may contain literal text and regular expression backreferences. If the substitution begins with the text `expr=` it is interpreted as an expression which allows access to environment variables and header values.

### Example

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

The character which is used to separate (or "delimit") the various parts of the substitution string is referred to as the "delimiter", and it is most common to use a slash for this purpose.

If either the pattern or the substitution contain a slash character then an alternative delimiter may be used to make the directive more readable:

### 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>

When using an expression for the substitution, regular expression backreferences must be backslash ('\') escaped as illustrated in the example below:

### Expression Example

    <Location "/">
        AddOutputFilterByType SUBSTITUTE text/html
        Substitute "s/example.com/expr=%{HTTP:HOST}/i"
        Substitute "s/Hello, (\S+)/expr=Hello from %{REQUEST_URI}, \$1/i"
    </Location>

### Expressions and caching

Caution must be exercised when performing substitutions that reference HTTP request headers. Because this module operates after response headers have been sent, the expression parser cannot add referenced HTTP request headers to the outgoing Vary header.

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_substitute` 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.

## SubstituteInheritBefore Directive ¶

Description: | Change the merge order of inherited patterns
Syntax: | `SubstituteInheritBefore on|off`
Default: | `SubstituteInheritBefore on`
Context: | directory, .htaccess
Override: | FileInfo
Status: | Extension
Module: | mod_substitute
Compatibility: | Available in httpd 2.4.17 and later

Whether to apply the inherited `Substitute` patterns first (`on`), or after the ones of the current context (`off`). The latter was the default in versions 2.4 and earlier, but changed starting with 2.5, hence `SubstituteInheritBefore` set to `off` allows to restore the legacy behavior. `SubstituteInheritBefore` is itself inherited, hence contexts that inherit it (those that don't specify their own `SubstituteInheritBefore` value) will apply the closest defined merge order.

## SubstituteMaxLineLength Directive ¶

Description: | Set the maximum line size
Syntax: | `SubstituteMaxLineLength bytes(b|B|k|K|m|M|g|G)`
Default: | `SubstituteMaxLineLength 1m`
Context: | directory, .htaccess
Override: | FileInfo
Status: | Extension
Module: | mod_substitute
Compatibility: | Available in httpd 2.4.11 and later

The maximum line size handled by `mod_substitute` is limited to restrict memory use. The limit can be configured using `SubstituteMaxLineLength`. The value can be given as the number of bytes and can be suffixed with a single letter `b`, `B`, `k`, `K`, `m`, `M`, `g`, `G` to provide the size in bytes, kilobytes, megabytes or gigabytes respectively.

### Example

    <Location "/">
        AddOutputFilterByType SUBSTITUTE text/html
        SubstituteMaxLineLength 10m
        Substitute "s/foo/bar/ni"
    </Location>