Ruby, weird substitution

For example:

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str1 = "pppp(m)pppp"
str2 = "(m)"
str1 = str1.sub(/#{str2}/, "<>#{str2}<>")

I will got this:

"pppp(<>(m)<>)pppp"

I expected to get this:

"pppp<>(m)<>pppp"

Why it's happening and how to avoid this?

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In ( and ) have a special meaning in regexen and do not actually match the characters ( and ). The regex /(m)/ will match any m whether or not it is enclosed in parentheses (and if it is, it won't match the parentheses).

To match literal parentheses use \( and \) - or in a case like this where you're interpolating a string, you can just use Regexp.escape on the string, i.e. /#{ Regexp.escape(str2) }/.

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The regular expression is viewing the "(m)" as a capture group because the parenthesis are operators in regular expressions to get a literal "(m)" you need to use the escape char \ ["\(m\)"].