fix false positive in is_date_format detection.
According to the spec _
(underline) denotes a skip the width of the next character.
So the underline should be added to the negative lookbehind in the regex.
Resolves #2014 (closed).
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According to the spec _
(underline) denotes a skip the width of the next character.
So the underline should be added to the negative lookbehind in the regex.
Resolves #2014 (closed).