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20130218b

Certain characters

Write a function with the following signature:

valid(text, chars="ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")

The function should return a (possibly empty) string, which contains only those characters from "text" (first parameter) that are present in "chars".

Examples:

valid("Barking!")                              -> "B"
valid("KL754", "0123456789")                   -> "754"
valid("BEAN", "abcdefghijklmnopqrstuvwxyz")    -> ""