created: 20140210141217955
list: [[Filter Expression]] [[Filter Run]] [[Filter Step]] [[Filter Parameter]] [[Filter Whitespace]]
modified: 20230710074340943
tags: Filters
title: Filter Syntax
type: text/vnd.tiddlywiki

<<.preamble """[[Filters]] follow a grammar that is presented here, using [[railroad diagrams|Railroad Diagrams]], for those who find formal syntax descriptions helpful. However, you can [[learn to write filters|Introduction to filter notation]] without needing to understand this group of tiddlers.""">>

A <<.def filter>> is a pipeline for transforming an <<.def input>> into an <<.def output>>. Both the input and the output are [[ordered sets of titles|Title Selection]] of tiddlers and fields.

Filters are ''expressions'' constructed from smaller building blocks, called ''runs'', which are built using ''steps''. Eeach 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.

The "Filter Syntax" description starts with:

<$railroad text="""
\start double
\end double
[[<"Filter Expression">|"Filter Expression"]]
"""/>

<<.tip "The railroad boxes as the one above can be used to navigate">>