I know a little about automata theory and formal languages but if you're taking about how to use regex in programming languages (such as Regex class in Java), I don't know how to use it. But if you're taking about this, for example:
L = {w element of sigma star | length of w is even}
in regex is: L(((a+b)(a+b))*)
then I might be able to help.
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Ang Akong Balay