Filters follow a grammar that is presented here, using railroad diagrams, for those who find formal syntax descriptions helpful. However, you can learn to write filters without needing to understand this group of tiddlers.

A filter is a pipeline for transforming an input into an output. Both the input and the output are ordered sets of titles of things like tiddlers and fields.

Filters are expressions constructed from smaller building blocks, called runs and steps, each of which also transforms an input to an output.

A filter starts with an empty output. Its runs are processed from left to right, progressively modifying the output.

Here are details of the various building blocks involved:

• Filter Expression
• Filter Run
• Filter Step
• Filter Parameter
• Filter Whitespace
• Cascade Filter Run Prefix
• Cascade Filter Run Prefix (Examples)
• Filter Filter Run Prefix
• Filter Filter Run Prefix (Examples)
• Filter Run Prefix (Examples)
• Intersection Filter Run Prefix
• Intersection Filter Run Prefix (Examples)
• Map Filter Run Prefix
• Map Filter Run Prefix (Examples)
• Named Filter Run Prefix
• Reduce Filter Run Prefix
• Reduce Filter Run Prefix (Examples)
• Sort Filter Run Prefix
• Sort Filter Run Prefix (Examples)
• Then Filter Run Prefix