The RewriteMap directive defines a Rewriting Map which can be used inside rule substitution strings by the mapping-functions to insert/substitute fields through a key lookup. The source of this lookup can be of various types.

The MapName is the name of the map and will be used to specify a mapping-function for the substitution strings of a rewriting rule via one of the following constructs:

${ MapName : LookupKey }
${ MapName : LookupKey | DefaultValue }

When such a construct occurs, the map MapName is consulted and the key LookupKey is looked-up. If the key is found, the map-function construct is substituted by SubstValue. If the key is not found then it is substituted by DefaultValue or by the empty string if no DefaultValue was specified. Empty values behave as if the key was absent, therefore it is not possible to distinguish between empty-valued keys and absent keys.

For example, you might define a RewriteMap as:

RewriteMap examplemap "txt:/path/to/file/map.txt"

You would then be able to use this map in a RewriteRule as follows:

RewriteRule "^/ex/(.*)" "${examplemap:$1}"

The following combinations for MapType and MapSource can be used:

txt

A plain text file containing space-separated key-value pairs, one per line. (Details ...)

rnd

Randomly selects an entry from a plain text file (Details ...)

dbm

Looks up an entry in a dbm file containing name, value pairs. Hash is constructed from a plain text file format using the httxt2dbm utility. (Details ...)

int

One of the four available internal functions provided by RewriteMap : toupper, tolower, escape or unescape. (Details ...)

prg

Calls an external program or script to process the rewriting. (Details ...)

dbd or fastdbd

A SQL SELECT statement to be performed to look up the rewrite target. (Details ...)

Further details, and numerous examples, may be found in the RewriteMap HowTo