"Did the population of the city decrease?"

[Menegobarba]

Why şehrin and not şehirin? Is it irregular or there is a rule?

[Selcen_Ozturk]

irregular and there is no rule

[RiaArik]

Some words, such as şehir, lose the vowel before the final letter (or it is transferred to the end of the word) when they are part of a possessive construction. My city=benim şehrim; your city=şehrin; his/her city=şehri, etc. Sorry, I can't think of the other Turkish words like this, off the top of my head. Will add more if I do. As I recall, the words are from Arabic.

[Yomalyn]

• oğul = son // oğlum = my son
• fikir = idea // fikrim = my idea
• isim = name // ismim = my name

There are quite a few, actually. More examples HERE

This site doesn't mention Arabic origin, so thanks for pointing that out! I did recognize a couple, but it's nice to know there is a common theme to an otherwise arbitrary list! :-)