All of a sudden the need for proper formal documentation has become a thing in our organization. A lot of work I do are enhancements and addition to an existing system. Generally a quick fix or a new method, etc.I am just not sure what kind of diagrams I should/could use for small projects. For instance I am adding a new functionality to an existing system that verifies format of a field and they want proper Component design description with diagrams for a simple regex that verifies format of one field. In the past we only did pseudocode and that was enough for such projects as the system is somewhat legacy with very little original documentation. 
UML diagrams for small projects
design;documentation
Your main problem seems to be that UML diagrams document systems, not projects. It is indeed completely nonsensical to demand separate UML documentation for a project that consists of adding a small amount of functionality to an existing large system. Talk to your management about how this discrepancy can be resolved. If all they want is some sort of formal documentation, propose a standardized form to handle such small change projects (with entries such as system to be changed, summary of change, affected assets, etc.). If they insist on UML purely for buzzword compatibility, a use case document (not diagram) is quite similar to that and may satisfy the demand.Alternatively or additionally, you may want to consider creating some high-level UML documentation for those legacy projects and integrate future changes in that.