Postgresql function similar to Find() in Excel? -

how can find position of string starting specified index in postgresql-9.3+

example

find('ranranran','an',1) should output 2 find('ranranran','an',3)  => 5  find('ranranran','an',6)  => 8 

i had found answer this, want post it.

strpos(substr('withintext',pos),'texttofind') + (pos-1)