How can I write a Python lexical analysis program that implements a simple scanner for a source file given as a command$-$line argument?

With an example token format:

keyword $->$ if $|$ then $|$ else $|$ begin $|$ end

identifier $->$ character $|$ character identifier

integer $->$ digit $|$ digit integer

real $->$ integer.integer

special $->(|)|[|]|+|-|=|,|$ ;

digit $->0|1|2|3|4|5|6|7|8|9$

character $->$ a$|$b$|$c $...|$z$|$A$|$B$|$C $...|$Z