This page transcribes words to and from Received English (RP) pronunciation, which is the pronunciation scheme most dictionaries use. You can type in any word in either English (Roman alphabet) letters or phonemic script, and it will do its best to transcribe it for you. It is based on data provided by the Regular English Pronunciation project.
If you are a student or teacher of English as a second language, or anyone interested in the phonology of English, it is highly recommended that you learn the phonemic chart.
It looks like you want to transcribe from English spelling to phonemic script.
This is how to write REX in phonemic script:
This form will allow you to transcribe any word to or from phonemic script. Just type it in and search.