JavaScript/REGEX: remove all words that do not begin with xyz

Using Regular Expressions and JavaScript, how can I remove all words that do not begin with xyz?
var str = 'xyz1 xyz2 xyzzy hello world xyz555 xyzttt';
str = str.replace(???????????????????????????????????);
alert(str); // should be: hello world

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LVL 16
hankknightAsked:
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Terry WoodsIT GuruCommented:
Going by the title of the question:
var str = 'xyz1 xyz2 xyzzy hello world xyz555 xyzttt';
str = str.replace(/\b(?!xyz)\w+/g, '');
alert(str);

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Result is:
xyz1 xyz2 xyzzy   xyz555 xyzttt

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Going by the comment in your starting code:
var str = 'xyz1 xyz2 xyzzy hello world xyz555 xyzttt';
str = str.replace(/\bxyz\w+/g, '');
alert(str); // should be: hello world

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Result is:
hello world

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Terry WoodsIT GuruCommented:
Let me know if you want the space for any invalid words removed too.
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David S.Commented:
You may be other-thinking it, Terry. You do want to match the "xyz". Also "\b" won't work as intended here because, at least in JavaScript RegEx, a hyphen is counted as a word-boundary.
str = str.replace(/(^|\s+)xyz\S*/g, '');

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Regular Expressions

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