[[analysis-porterstem-tokenfilter]]
=== Porter Stem Token Filter

A token filter of type `porter_stem` that transforms the token stream as
per the Porter stemming algorithm.

Note, the input to the stemming filter must already be in lower case, so
you will need to use
<<analysis-lowercase-tokenfilter,Lower
Case Token Filter>> or
<<analysis-lowercase-tokenizer,Lower
Case Tokenizer>> farther down the Tokenizer chain in order for this to
work properly!. For example, when using custom analyzer, make sure the
`lowercase` filter comes before the `porter_stem` filter in the list of
filters.