Question

Is there an existing solution to create a regular expressions dynamically out of a given date-time format pattern? The supported date-time format pattern does not matter (Joda DateTimeFormat, java.text.SimpleDateTimeFormat or others).

As a specific example, for a given date-time format like dd/MM/yyyy hh:mm, it should generate the corresponding regular expression to match the date-times within the specified formats.

Answer 1

I guess you have a limited alphabet that your time formats can be constructed of. That means, 'HH' would always be ‘hours’ on the 24-hour clock, 'dd' always the day with leading zero, and so on.

Because of the sequential nature of a time format, you could try to tokenize a format string of 'dd/mm/yyyy HH:nn' into an array ['dd', '/', 'mm', '/', 'yyyy', ' ', 'HH', ':', 'nn']. Then go ahead and form a pattern string from that array by replacing 'HH' with '([01][0-9]|2[0-3])' and so on. Preconstruct these pattern atoms into a lookup table/array. All parts of your array that are not in the lookup table are literals. Escape them to according regex rules and append them to you pattern string.

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EDIT: As a side effect for a regex based solution, when you put all regex ‘atoms’ of your lookup table into parens and keep track of their order in a given format string, you would be able to use sub-matches to extract the required components from a match and feed them into a CreateDate function, thus skipping the ParseDate part altogether.