How to change this Ruby regex to also include underlines?

At the moment I am using this line to take a string and extract only the letters from it:

string.scan(/[a-zA-Z]/).to_s

How do I modify this so that the underline character, "开发者_StackOverflow社区_", is also included? Thanks for reading.

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Add it within the brackets (the range IIRC).

string.scan(/[a-zA-Z_]/).to_s

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Alternative version

string.scan(/[a-z_]/i).to_s