| Horde_Constraint | Interface for constraints. |
| Horde_Constraint_AlwaysFalse | Always returns false |
| Horde_Constraint_AlwaysTrue | Always returns true |
| Horde_Constraint_And | Represents a collection of constraints, if one is false, this collection will evaluate to false |
| Horde_Constraint_Coupler | Interface for grouped (compound, coupled) constraints. |
| Horde_Constraint_IsEqual | Checks for equality |
| Horde_Constraint_IsInstanceOf | Checks for an instance of a class |
| Horde_Constraint_Not | Negates another constraint |
| Horde_Constraint_Null | Checks if the value is null |
| Horde_Constraint_Or | Represents a collection of constraints, if any are true, the collection will evaluate to true. |
| Horde_Constraint_PregMatch | Matches against a PCRE regex |