search for: tsibi

Hello, Is there a function for searching a string for a given character? For example, I have transcriptions like tsibi tSibi tibi kibi I want to search the character object for the presence of 's' or 'S' or 'k'. I thought perhaps pmatch(c("k", "S", "s"), x) would work for me, but it wants to compare from the start of the string and returns NA for "S"...