a - Matches the string a
a+b - Matches the string ab
a|b - Matches either the string a and the string b
a* - Matches the empty string, a, aa, aaa, aaaa, etc.
(a|b)* - Matches any string of a's and b's.
(a+a)* - Matches the empty string, aa, aaaa, aaaaaa, etc.

-input
prefix | infix | postfix
a        a       a
-output
a
-input
prefix | infix | postfix
*a       a*      a*
-output

a
aa
aaa
aaaa
...
-input
prefix | infix | postfix
*|ab     (a|b)*  ab|*
-output
a
b

ab
ba
aa
bb
...
-input
prefix    | infix             | postfix
*|+a+aa+bb  ((a+(a+a))|(b+b))*  bb+aa+a+|*

aaa
bb
aaabb
bbaaa
aaabbaaa
bbaaabb
bbbbaaa
...