How to use Regular Expressions or Regex?

Let's go through one simple example from the amazon site to understand regular expressions.

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Real-time Scenario: Suppose, you are testing the Amazon site for your test automation practice. How do we take a number that has $22.09(example below)? How do we convert that into a number? And, what if, the price is shown into £22.09, Yen22.09 to those country specifics?