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

--

There are 10 kinds of people -- those that understand binary, and those that don't.

In reply to Re: Regular Expressions by mephit
in thread Parenthesis usage in Regular Expressions by WarrenBullockIII

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.