Characteristics of Migemo
Tolerate inconsistency of representations
e.g., konnnitiha→ 「今日は」 「こんにちは」
Match all homonyms
e.g., kikai→「機械」 「機会」 「奇怪」 「器械」
Lookup dictionaries dynamically
Pre-compiled regular expressions 
For short patterns like "s", "k", etc.
Hard to deal with Unknown Words
Words which not defined in dictionaries
Just like Kana-Kanji conversion system