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My question essentially boils down to the following: Is it always possible to
assign a (regular / irregular) reading to a single kanji within some word? It
is related to this post: [Kanji that don't use their specified
readings](https://japanese.stackexchange.com/questions/38728/kanji-that-dont-
use-their-specified-readings):

The context is somewhat technical: I'm writing software that should map a
kanji to its reading within a certain expression. I'd like to collect all
irregular readings for the kanji and want to know, which irregularities are
most common. I'm using a database of all official on/kun-readings and Jim
Breen's dictionary and for this: For each of its ~190.000 entries, I try to
match the native (kanji) expression character-wise to its reading defined in
the dictionary. I reuse all irregular readings that could be uniquely
determined (i.e. if a single kanji remains unresolved in an expression, I use
the remaining reading part as irregular reading for this kanji. E.g. お兄さん ->
兄's reading in this case is にい). I'm not sure, if this is always correct
though, here are some examples:

今日 -> きょう (is 今[きょ]日[う] correct?)

明日 -> あした (is 明[あ]日[した]correct?)

越幾斯 -> えきす (is 越[え]幾[き]斯[す] correct?)

Around 300 (exotic?) entries remain, where it is not possible to uniquely
determine the reading of each kanji. How about the following exemplary ones:

七週の祭 -> ななまわりのいわい

海月 -> くらげ

Do these ones have to be learned as a compound? Thanks in advance for your
help!
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2019-07-16T14:40:34.650
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2019-07-16T16:05:38.770
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question
kanji readings
Reading per kanji / irregular readings
The short answer is no, it's not possible.  
While most kanji can be mapped to their most common readings, as you have
already shown in the question, there are exceptions where it's not so simple.  
今日 -> 今{きょ} 日{う}  
明日 -> 明{あ} 日{した}

But even in this case they can be mapped arbitrarily, right? Because it is
arbitrary, asking whether it is correct or not becomes moot. Note, however,
that there is no other way to divide きょう than into `きょ and う` since `き and ょう`
is just nonsensical (I'm assuming you don't want a "null" kanji). So while
these _can_ be mapped like you have them, any mapping is going to be
arbitrary.

You have also already mentioned 熟字訓{じゅくじくん} like

> 海月{くらげ}

Again, there is no "correct" way to map these because these kanji don't have
those readings, irregular or not.

The real reason I said it is not possible though, is the existence of 3-kanji
熟字訓{じゅくじくん} with 2-kana readings.  
As in

> 大口魚{たら}  
>  桃花鳥{とき}  
>  百舌鳥{もず}  
>  五十日{いか}

Granted this mostly occurs with last names, place names, and (usually)
alternate ways of writing animal names.

Plus, you have to figure out what to do with words where the same kanji
compound has 2 different readings (and meanings) depending on context.  
As in

> 人気{にんき} 人気{ひとけ}

And then you have words where the order of the kanji is opposite the reading  
As in

> 不忍{しのばず}

It's the first kanji 不 that represents the last kana ず.

This word also leads to the last category of readings problematic to mapping I
can think of, hidden kana. Most often の, but there are others.  
As in

> 不忍池{しのばずのいけ}

It's not possible to map the second の anywhere.

I think you could go a long way with just the regular/irregular readings, so
for practical purposes it may actually be possible. But, even some of the most
common words will give you many exceptions you would have to handle on a case-
by-case basis.
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answer