-ず and -ぬ are two alternatives to the negative form -ない / -ません. But I noticed that depending on the word, it's either -ず or -ぬ, although it seems like some words can take both suffixes. Some examples I have encountered:
-ず
知らず (lyric in song "touch"タッチ" by Younha)
わからず (lyric in song "aisuru mono;愛するもの" by Angela Aki/アンジェラ・アキ)
せず (song title: "yudan sezu ni ikou;油断せずに行こう")
なれず (lyric in song "mirae he;未来へ" by Kiroro/キロロ)
-ぬ
気づかぬ (lyric in song "arigatou;ありがとう..." by KokiaKOKIA)
届かぬ (song title: "todokanu omoi;届かぬ想い")
ならぬ (lyric in song "shunrai;春雷" by YellowYeLLOW Generation)
both -ず and -ぬ
とらず / とらぬ
できず / できぬ
So are there rules for choosing between -ず and -ぬ?