add japanese phrase preprocessing

Code adapted from GSOC code by @miku.

src/nominatim_api/query_preprocessing/split_japanese_phrases.py

@@ -0,0 +1,61 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2025 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+This file divides Japanese addresses into three categories:
+prefecture, municipality, and other.
+The division is not strict but simple using these keywords.
+"""
+from typing import List
+import re
+
+from .config import QueryConfig
+from .base import QueryProcessingFunc
+from ..search.query import Phrase
+
+MATCH_PATTERNS = [
+    r'''
+                (...??[都都道府県縣])            # [group1] prefecture
+                (.+?[市区區町村])              # [group2] municipalities (city/wards/towns/villages)
+                (.+)                         # [group3] other words
+                ''',
+    r'''
+                (...??[都都道府県縣])            # [group1] prefecture
+                (.+)                         # [group3] other words
+                ''',
+    r'''
+                (.+?[市区區町村])              # [group2] municipalities (city/wards/towns/villages)
+                (.+)                         # [group3] other words
+                '''
+]
+
+
+class _JapanesePreprocessing:
+
+    def __init__(self, config: QueryConfig) -> None:
+        self.config = config
+
+    def split_phrase(self, phrase: Phrase) -> Phrase:
+        """
+        This function performs a division on the given text using a regular expression.
+        """
+        for pattern in MATCH_PATTERNS:
+            result = re.match(pattern, phrase.text, re.VERBOSE)
+            if result is not None:
+                return Phrase(phrase.ptype, ':'.join(result.groups()))
+
+        return phrase
+
+    def __call__(self, phrases: List[Phrase]) -> List[Phrase]:
+        """Split a Japanese address using japanese_tokenizer.
+        """
+        return [self.split_phrase(p) for p in phrases]
+
+
+def create(config: QueryConfig) -> QueryProcessingFunc:
+    """ Create a function of japanese preprocessing.
+    """
+    return _JapanesePreprocessing(config)