It cannot be done using a GREP style, only with Find/Change.
GREP styles allow you to apply a character style to text that matches your pattern, but cannot add, remove, or rearrange the text.
Iam Sorry for the wrong question its not changing by grep style. Is it possible that Dateformat change by grep using find and change.
In your example above you are reversing the direction of the slashes is that what you want to do?
If so, (?<=\d)/(?=\d) will find the forward slashes between two digits, and you can enter the backslash in the change field.
To tell the true, in this particular case, you could use a GREP styles to change (reformat) this:
You would need 2 charStyles:
1. "slash" - set tracking -150 and skew -30; (perhaps slightly modify depending on font)
2. "space" - set tracking +150 (perhaps slightly modify depending on font)
as a nested GREP styles use:
charStyle "slash" with condition (?<=\d)/
charStyle "space" with condition \d(?=/)
Applying this paraStyle to 01/02/2012 should display it as 01\02\2012
just for fun