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copied!<p>This actually is something that regexes are good at; making an assumption, moving forward with it, then backtracking if necessary to get a successful match.</p> <pre><code>s{ \A ( 1[0-2] | 0?[1-9] ) [-/ ]? ( 3[01] | [12][0-9] | 0?[1-9] ) [-/ ]? ( (?: [0-9]{2} ){1,2} ) \z } { sprintf '%02u/%02u/%04u', $1, $2, ( length $3 == 4 ? $3 : 2000+$3 ) }xe; </code></pre> <p>The range checks present, while not determined by the value of the month, should be sufficient to pick a good date from the ambiguous cases (where there is a good date).</p> <p>Note that it is important to try two digit month and days first; otherwise 111111 becomes 1-1-1111, not the presumably intended 11-11-11. But this means 11111 will prefer to be 11-1-11, not 1-11-11.</p> <p>If a valid day of month check is needed, it should be performed after reformatting.</p> <p>Notes:</p> <p><code>s{}{}</code> is a substitution using curly braces instead of / to delimit the parts of the regex to avoid having to escape the /, and also because using paired delimiters allows opening and closing both the pattern and replacement parts, which looks nice to me.</p> <p><code>\A</code> matches the start of the string being matched; <code>\z</code> matches the end. <code>^</code> and <code>$</code> are often used for this, but can have slightly different meanings in some cases; I prefer these since they always only mean one thing.</p> <p>The x flag on the end says this is an extended regex that can have extra whitespace or comments that are ignored, so that it is more readable. (Whitespace inside a character class isn't ignored.) The e flag says the replacement part isn't a string, it is code to execute.</p> <p><code>'%02u/%02u/%02u'</code> is a printf format, used for taking values and formatting them in a particular way; see <a href="http://perldoc.perl.org/functions/sprintf.html" rel="nofollow">http://perldoc.perl.org/functions/sprintf.html</a>.</p>

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