### Converting strings into expressions

Hey everyone,
This is probably pretty straightforward, but I couldn't find any straightforward answers with google searching. I'm looking for a function that works in the same way as python's str() and float() functions, but turns variables (especially strings) into sage's different classes of variables, like expressions.
For example, if I have string variable:
expression = "x1^2*x2*x3*4"
and x1, x2, and x3 are already declared as variables, how can I turn that variable into a sage expression so it will work with things like the coefficient() function?
Thanks!