niho

niho is a command-line tool for converting romanized Japanese text to Japanese characters.

$ echo _niho ha, Ro-ma ji_ wo nihongo_ni henkan_surutameno Tu-ru desu. | niho
nihoは、ローマ字を日本語に変換するためのツールです。

Installation

$ cargo install niho
A command-line tool for converting romanized Japanese text to Japanese characters

Usage: niho [OPTIONS]

Options:
      --version                Print version
  -h, --help                   Print help ('--help' for full help, '-h' for summary)
  -t, --tokenize               Output tokenized input as JSON instead of converting to Japanese
  -d, --dictionary-file <PATH> Path to dictionary file [env: NIHO_DICTIONARY_FILE]

Basic Syntax

niho converts romanized Japanese text to Japanese characters using the following syntax:

Input Text Types

• Regular text: Converted to hiragana (e.g., konnnichiwa → こんにちは)
• Capitalized text: Converted to katakana (e.g., Ko-hi- → コーヒー)
• Text ending with _: Converted to kanji using dictionary lookup (e.g., nihongo_ → 日本語)
• Text wrapped in ___: Kept as raw text without conversion (e.g., ___Hello___ → Hello)
• Text prefixed with _: Kept as raw text until whitespace (e.g., _Hello desu → Hello です)

Examples

# Convert hiragana
$ echo konnnichiwa | niho
こんにちは

# Convert katakana (use uppercase)
$ echo Ko-hi- | niho
コーヒー

# Convert kanji (use underscore suffix)
$ echo nihongo_ | niho
日本語

# Mix different types
$ echo watashi ha Ko-hi- wo nomimasu | niho
わたしはコーヒーをのみます

# Keep raw text
$ echo ___English to ___ nihongo_ | niho
English to 日本語

Dictionary Format

The dictionary is stored in a JSONL (JSON Lines) format, where each line contains a JSON object representing a character or word mapping. The dictionary contains three types of entries:

• hiragana: Maps romanized text to hiragana characters
• katakana: Maps romanized text to katakana characters
• kanji: Maps romanized words to kanji characters

Example entries:

{"type": "hiragana", "from": "ka", "to": "か"}
{"type": "katakana", "from": "ka", "to": "カ"}
{"type": "kanji", "from": "nihongo", "to": "日本語"}

The default dictionary can be found at default-dic.jsonl.

Kanji and Unknown Word Handling

For kanji conversion (text ending with _), the tool performs a direct dictionary lookup:

• Found words: If the romanized text exists in the kanji dictionary, it's converted to the corresponding kanji (e.g., nihongo_ → 日本語)
• Unknown words: If no mapping is found, the text is wrapped in angle brackets to indicate it's unrecognized (e.g., unknown_ → <unknown>)