!!! Overview[1]
[{$pagename}] is an [LDAP SearchFilter] defined in [RFC 4511] which states:

"An [{$pagename}] filter is __TRUE__ when there is a value of the attribute type or subtype for which some locally-defined approximate matching algorithm (e.g., spelling variations, phonetic match, etc.) returns TRUE.  If a value matches for equality, it also satisfies an approximate match.  If approximate matching is not supported for the attribute, this filter item should be treated as an [EQUALITY]."

%%information
None of the [RFCs] define a [MatchingRule] for [APPROXIMATE]
%%

Although [{$pagename}] is mentioned in [RFC 4511] in the description of [LDAP SearchFilters] none of the [RFCs] define a [MatchingRule] for [APPROXIMATE]. This implies that the operations and specifics of how an [APPROXIMATE] [MatchingRule] works is left to the [LDAP Server Implementations] on how it works.
	
An [{$pagename}] filter may be used to determine whether an entry contains at least one value for a specified attribute that is approximately equal to a given value. The LDAP specifications do not define what exactly "approximately equal to" means, so that is left up to individual [LDAP Server Implementations] to determine. Many servers use a "sounds like" mechanism with an algorithm based on Soundex or one of the Metaphone variants.

The string representation of an [{$pagename}] filter is constructed as follows:
* An open parenthesis
* The attribute description (potentially including attribute options)
* A tilde character An equal sign The value to compare (aka the assertion value)
* A close parenthesis

For [Example], it might be reasonable to expect a filter of 
%prettify 
{{{
(givenName~=John)
}}} /%
to match entries with givenName values of either John or Jon.

Although it seems like a significant oversight or omission, the LDAP specifications do not make any provision for [{$pagename}] [matching Rules]. A number of individual [LDAP Server Implementations] provide this capability anyway so that it may be possible to configure the approximate match behavior on a per-attribute basis, but the inconsistency of approximate matching capabilities between server implementations makes approximate matching something that is often avoided in LDAP-enabled applications.

An approximate search filter is a type of [LDAP SearchFilter] that can be used to identify entries that contain a value for a given attribute that is approximately equal to a given assertion value.  The server will use an approximate [matchingRule] to make the determination.

!! Observations
We have seen [X-APPROX] used to indicate that [{$pagename}] [matching Rules] be applied.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]