Without the parens, the alternation will appear to be "greedy." The perlre man page says:

You can specify a series of alternatives for a pattern using "|" to separate them, so that `fee|fie|foe' will match any of "fee", "fie", or "foe" in the target string (as would `f(e|i|o)e'). The first alternative includes everything from the last pattern delimiter ("(", "[", or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next pattern delimiter. That's why it's common prac­tice to include alternatives in parentheses: to minimize confusion about where they start and end.

Ie., without the parens, it's like checking for either /\bfred/ or /wilma\s+flintstone\b/, which isn't What You Want, and the string "I am fred" will match successfully, even though there's no "flintstone" to be found anywhere in the string.

In short, use parens in an alternating pattern to avoid confusion. HTH

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