Tokenizers

The tokenizer module in Nominatim is responsible for analysing the names given to OSM objects and the terms of an incoming query in order to make sure, they can be matched appropriately.

Nominatim offers different tokenizer modules, which behave differently and have different configuration options. This sections describes the tokenizers and how they can be configured.

Important

The use of a tokenizer is tied to a database installation. You need to choose and configure the tokenizer before starting the initial import. Once the import is done, you cannot switch to another tokenizer anymore. Reconfiguring the chosen tokenizer is very limited as well. See the comments in each tokenizer section.

Legacy tokenizer

The legacy tokenizer implements the analysis algorithms of older Nominatim versions. It uses a special Postgresql module to normalize names and queries. This tokenizer is currently the default.

To enable the tokenizer add the following line to your project configuration:

NOMINATIM_TOKENIZER=legacy

The Postgresql module for the tokenizer is available in the module directory and also installed with the remainder of the software under lib/nominatim/module/nominatim.so. You can specify a custom location for the module with

NOMINATIM_DATABASE_MODULE_PATH=<path to directory where nominatim.so resides>

This is in particular useful when the database runs on a different server. See Advanced installations for details.

There are no other configuration options for the legacy tokenizer. All normalization functions are hard-coded.

ICU tokenizer

Danger

This tokenizer is currently in active development and still subject to backwards-incompatible changes.

The ICU tokenizer uses the ICU library to normalize names and queries. It also offers configurable decomposition and abbreviation handling.

How it works

On import the tokenizer processes names in the following four stages:

  1. The Normalization part removes all non-relevant information from the input.
  2. Incoming names are now converted to full names. This process is currently hard coded and mostly serves to handle name tags from OSM that contain multiple names (e.g. Biel/Bienne).
  3. Next the tokenizer creates variants from the full names. These variants cover decomposition and abbreviation handling. Variants are saved to the database, so that it is not necessary to create the variants for a search query.
  4. The final Tokenization step converts the names to a simple ASCII form, potentially removing further spelling variants for better matching.

At query time only stage 1) and 4) are used. The query is normalized and tokenized and the resulting string used for searching in the database.

Configuration

The ICU tokenizer is configured using a YAML file which can be configured using NOMINATIM_TOKENIZER_CONFIG. The configuration is read on import and then saved as part of the internal database status. Later changes to the variable have no effect.

Here is an example configuration file:

normalization:
    - ":: lower ()"
    -  > 'ss'" # German szet is unimbigiously equal to double ss
transliteration:
    - !include /etc/nominatim/icu-rules/extended-unicode-to-asccii.yaml
    - ":: Ascii ()"
variants:
    - language: de
      words:
        - ~haus => haus
        - ~strasse -> str
    - language: en
      words: 
        - road -> rd
        - bridge -> bdge,br,brdg,bri,brg

The configuration file contains three sections: normalization, transliteration, variants.

The normalization and transliteration sections each must contain a list of ICU transformation rules. The rules are applied in the order in which they appear in the file. You can also include additional rules from external yaml file using the !include tag. The included file must contain a valid YAML list of ICU rules and may again include other files.

Warning

The ICU rule syntax contains special characters that conflict with the YAML syntax. You should therefore always enclose the ICU rules in double-quotes.

The variants section defines lists of replacements which create alternative spellings of a name. To create the variants, a name is scanned from left to right and the longest matching replacement is applied until the end of the string is reached.

The variants section must contain a list of replacement groups. Each group defines a set of properties that describes where the replacements are applicable. In addition, the word section defines the list of replacements to be made. The basic replacement description is of the form:

<source>[,<source>[...]] => <target>[,<target>[...]]

The left side contains one or more source terms to be replaced. The right side lists one or more replacements. Each source is replaced with each replacement term.

Tip

The source and target terms are internally normalized using the normalization rules given in the configuration. This ensures that the strings match as expected. In fact, it is better to use unnormalized words in the configuration because then it is possible to change the rules for normalization later without having to adapt the variant rules.

Decomposition

In its standard form, only full words match against the source. There is a special notation to match the prefix and suffix of a word:

- ~strasse => str  # matches "strasse" as full word and in suffix position
- hinter~ => hntr  # matches "hinter" as full word and in prefix position

There is no facility to match a string in the middle of the word. The suffix and prefix notation automatically trigger the decomposition mode: two variants are created for each replacement, one with the replacement attached to the word and one separate. So in above example, the tokenization of "hauptstrasse" will create the variants "hauptstr" and "haupt str". Similarly, the name "rote strasse" triggers the variants "rote str" and "rotestr". By having decomposition work both ways, it is sufficient to create the variants at index time. The variant rules are not applied at query time.

To avoid automatic decomposition, use the '|' notation:

- ~strasse |=> str

simply changes "hauptstrasse" to "hauptstr" and "rote strasse" to "rote str".

Initial and final terms

It is also possible to restrict replacements to the beginning and end of a name:

- ^south => s  # matches only at the beginning of the name
- road$ => rd  # matches only at the end of the name

So the first example would trigger a replacement for "south 45th street" but not for "the south beach restaurant".

Replacements vs. variants

The replacement syntax source => target works as a pure replacement. It changes the name instead of creating a variant. To create an additional version, you'd have to write source => source,target. As this is a frequent case, there is a shortcut notation for it:

<source>[,<source>[...]] -> <target>[,<target>[...]]

The simple arrow causes an additional variant to be added. Note that decomposition has an effect here on the source as well. So a rule

- "~strasse -> str"

means that for a word like hauptstrasse four variants are created: hauptstrasse, haupt strasse, hauptstr and haupt str.

Reconfiguration

Changing the configuration after the import is currently not possible, although this feature may be added at a later time.