Using flex in c and regular expressions

I am trying to create a lexical analyzer for a compiler.But I have a problem using regular expressions to find things like keywords and real numbers.. for example some definitions :

id         [aA-zZ][aA-zZ-0-9_]* 

keyword    if|else|when|while

integer    [0-9]+

real       integer.integer

..There are some problems though,the analyzer cant get a keyword for example instead if i give the word 'else' it sees it as a id(I get a warning like rule cannot be matched too.

Also if I try to give a real number for example 1.2 the linker sees it as integer delimeter integer instead as a real.I am not good at regural expressions language though,..I thought for the real/integer distinction to put a rule like ("Read a number that doesn't end with . and it's an integer , else it's a number") but how can I put that in reg. language.