add documentation for postcode customization

docs/customize/Country-Settings.md

@@ -0,0 +1,149 @@
+# Customizing Per-Country Data
+
+Whenever an OSM is imported into Nominatim, the object is first assigned
+a country. Nominatim can use this information to adapt various aspects of
+the address computation to the local customs of the country. This section
+explains how country assignment works and the principal per-country
+localizations.
+
+## Country assignment
+
+Countries are assigned on the basis of country data from the OpenStreetMap
+input data itself. Countries are expected to be tagged according to the
+[administrative boundary schema](https://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative):
+a OSM relation with `boundary=administrative` and `admin_level=2`. Nominatim
+uses the country code to distinguish the countries.
+
+If there is no country data available for a point, then Nominatim uses the
+fallback data imported from `data/country_osm_grid.sql.gz`. This was computed
+from OSM data as well but is guaranteed to cover all countries.
+
+Some OSM objects may also be located outside any country, for example a buoy
+in the middle of the ocean. These object do not get any country assigned and
+get a default treatment when it comes to localized handling of data.
+
+## Per-country settings
+
+### Global country settings
+
+The main place to configure settings per country is the file
+`settings/country_settings.yaml`. This file has one section per country that
+is recognised by Nominatim. Each section is tagged with the country code
+(in lower case) and contains the different localization information. Only
+countries which are listed in this file are taken into account for computations.
+
+For example, the section for Andorra looks like this:
+
+```
+    partition: 35
+    languages: ca
+    names: !include country-names/ad.yaml
+    postcode:
+      pattern: "(ddd)"
+      output: AD\1
+```
+
+The individual settings are described below.
+
+#### `partition`
+
+Nominatim internally splits the data into multiple tables to improve
+performance. The partition number tells Nominatim into which table to put
+the country. This is purely internal management and has no effect on the
+output data.
+
+The default is to have one partition per country.
+
+#### `languages`
+
+A comma-separated list of ISO-639 language codes of default languages in the
+country. These are the languages used in name tags without a language suffix.
+Note that this is not necessarily the same as the list of official languages
+in the country. There may be officially recognised languages in a country
+which are only ever used in name tags with the appropriate language suffixes.
+Conversely, a non-official language may appear a lot in the name tags, for
+example when used as an unofficial Lingua Franca.
+
+List the languages in order of frequency of appearance with the most frequently
+used language first. It is not recommended to add languages when there are only
+very few occurrences.
+
+If only one language is listed, then Nominatim will 'auto-complete' the
+language of names without an explicit language-suffix.
+
+#### `names`
+
+List of names of the country and its translations. These names are used as
+a baseline. It is always possible to search countries by the given names, no
+matter what other names are in the OSM data. They are also used as a fallback
+when a needed translation is not available.
+
+!!! Note
+    The list of names per country is currently fairly large because Nominatim
+    supports translations in many languages per default. That is why the
+    name lists have been separated out into extra files. You can find the
+    name lists in the file `settings/country-names/<country code>.yaml`.
+    The names section in the main country settings file only refers to these
+    files via the special `!include` directive.
+
+#### `postcode`
+
+Describes the format of the postcode that is in use in the country.
+
+When a country has no official postcodes, set this to no. Example:
+
+```
+ae:
+    postcode: no
+```
+
+When a country has a postcode, you need to state the postcode pattern and
+the default output format. Example:
+
+```
+bm:
+    postcode:
+      pattern: "(ll)[ -]?(dd)"
+      output: \1 \2
+```
+
+The **pattern** is a regular expression that describes the possible formats
+accepted as a postcode. The pattern follows the standard syntax for
+[regular expressions in Python](https://docs.python.org/3/library/re.html#regular-expression-syntax)
+with two extra shortcuts: `d` is a shortcut for a single digit([0-9])
+and `l` for a single ASCII letter ([A-Z]).
+
+Use match groups to indicate groups in the postcode that may optionally be
+separated with a space or a hyphen.
+
+For example, the postcode for Bermuda above always consists of two letters
+and two digits. They may optionally be separated by a space or hyphen. That
+means that Nominatim will consider `AB56`, `AB 56` and `AB-56` spelling variants
+for one and the same postcode.
+
+Never add the country code in front of the postcode pattern. Nominatim will
+automatically accept variants with a country code prefix for all postcodes.
+
+The **output** field is an optional field that describes what the canonical
+spelling of the postcode should be. The format is the
+[regular expression expand syntax](https://docs.python.org/3/library/re.html#re.Match.expand) referring back to the bracket groups in the pattern.
+
+Most simple postcodes only have one spelling variant. In that case, the
+**output** can be omitted. The postcode will simply be used as is.
+
+In the Bermuda example above, the canonical spelling would be to have a space
+between letters and digits.
+
+!!! Warning
+    When your postcode pattern covers multiple variants of the postcode, then
+    you must explicitly state the canonical output or Nominatim will not
+    handle the variations correctly.
+
+### Other country-specific configuration
+
+There are some other configuration files where you can set localized settings
+according to the assigned country. These are:
+
+ * [Place ranking configuration](Ranking.md)
+
+Please see the linked documentation sections for more information.

docs/customize/Tokenizers.md

@@ -205,6 +205,14 @@ The following is a list of sanitizers that are shipped with Nominatim.
     rendering:
         heading_level: 6
 
+##### clean-postcodes
+
+::: nominatim.tokenizer.sanitizers.clean_postcodes
+    selection:
+        members: False
+    rendering:
+        heading_level: 6
+
 
 #### Token Analysis
 
@@ -222,8 +230,12 @@ by a sanitizer (see for example the
 The token-analysis section contains the list of configured analyzers. Each
 analyzer must have an `id` parameter that uniquely identifies the analyzer.
 The only exception is the default analyzer that is used when no special
-analyzer was selected. There is one special id '@housenumber'. If an analyzer
-with that name is present, it is used for normalization of house numbers.
+analyzer was selected. There are analysers with special ids:
+
+ * '@housenumber'. If an analyzer with that name is present, it is used
+   for normalization of house numbers.
+ * '@potcode'. If an analyzer with that name is present, it is used
+   for normalization of postcodes.
 
 Different analyzer implementations may exist. To select the implementation,
 the `analyzer` parameter must be set. The different implementations are
@@ -356,6 +368,14 @@ house numbers of the form '3 a', '3A', '3-A' etc. are all considered equivalent.
 
 The analyzer cannot be customized.
 
+##### Postcode token analyzer
+
+The analyzer `postcodes` is pupose-made to analyze postcodes. It supports
+a 'lookup' varaint of the token, which produces variants with optional
+spaces. Use together with the clean-postcodes sanitizer.
+
+The analyzer cannot be customized.
+
 ### Reconfiguration
 
 Changing the configuration after the import is currently not possible, although

docs/mkdocs.yml

@@ -28,6 +28,7 @@ pages:
         - 'Overview': 'customize/Overview.md'
         - 'Import Styles': 'customize/Import-Styles.md'
         - 'Configuration Settings': 'customize/Settings.md'
+        - 'Per-Country Data': 'customize/Country-Settings.md'
         - 'Place Ranking' : 'customize/Ranking.md'
         - 'Tokenizers' : 'customize/Tokenizers.md'
         - 'Special Phrases': 'customize/Special-Phrases.md'