Readable solution, but scans entire string: 

if (grep $_!=2, map length, /(:+)/g) {
    die
}

[download]

Stops at first error: 

if (/: (?: :: | (?<!::)(?!:) )/x) {
    die
}

[download]

In reply to Re: Regex Validation by ikegami
in thread Regex Validation by jimb

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