The $ symbol means the end of the line, and the | symbol means 'or'.

So this part of the regexp

($|(?=\w+:))
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translates to "match if either the end of line has been reached, OR if the next part (lookahead) matches one or more alphanumeric characters followed by a colon"

Without checking for the end of line, the last parameter would always be missed out (it would only match a parameter if it was followed by another one).

JJ