title: Using regexp_replace to remove letters from numbers
body: <p>I've got a bunch of data with altitude - some of it just numbers, some include meters at the end or '. I also have few ranges 1200-1300 etc (I guess it the second problem would have to be solved a different way). I tried experimenting with regexp_replace but [^a-z] doesn't seem to be working. 
  Any of you have a good idea on how to get rid of everything that's not a digit? Also, if you could recommend good website/book/course on how to clear data, I'd be much appreciated. Thanks! </p>