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Ruby Programming
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Alternation

We know that the vertical bar is special, because our line splitting pattern had to escape it with a backslash. That's because an unescaped vertical bar ``|'' matches either the regular expression that precedes it or the regular expression that follows it.

a = "red ball blue sky"
showRE(a, /d|e/) r<<e>>d ball blue sky
showRE(a, /al|lu/) red b<<al>>l blue sky
showRE(a, /red ball|angry sky/) <<red ball>> blue sky

There's a trap for the unwary here, as ``|'' has a very low precedence. The last example above matches ``red ball'' or ``angry sky'', not ``red ball sky'' or ``red angry sky''. To match ``red ball sky'' or ``red angry sky'', you'd need to override the default precedence using grouping.
Ruby Programming
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