Javascript Regex Expression Challenge

I am looking out for a regular expression that takes the following format

textfile_20060711_113441.txt

where
textfile = name of the file
2006 = year
07=month
11=day
11=hour
34=minute
41=seconds.
.txt is extension

The file always comes in the same format. however we need to implement checks so that user doesnt enter invalid month/day/hour/minute etc.

I have created the following regular expression
textfile_(?<Year>(?:d{4}|d{2}))(?x)(?<Month>d{1,2})(?<Day>d{1,2})_([01]?[0-9]|[2][0-3])([0-5][0-9])([0-5][0-9]).[a-zA-Z]{3}

Can anyone guide me as to how do i modify this regex to suit my requirement. This regex currently is not able to handle invalid months or days etc.

Also i do not know how to integrate this regex in my javascript. Pointers or examples will be helpful.