InfoPath Dev

I have read through most of these posts and am still having issues with getting a leading zero to work.

I have InfoPath 2010 form called CRequest3 (atttached)that is being pulled up and filled out in SharePoint.

I have a field called docSiteID it has to have 3 digits, if a 2 digit number is entered, a leading zero must be added to the field. -we use the data to run reports and a 3 digit number is a must otherwise it errors out.

I have tried many types of function from your sheets, I guess since I am so new at this I am not understanding how to fill out the boxes correctly. 

Please help me.

This can be done by adding two seperate rules to the text box that you are using. I've noticed this only works if the text box is set to a String type and not for Integer type text boxes. At least this is the case in Infopath 2007

Rule 1: Condition - If field1 matches the pattern \d (this means one integer)

           Action: Set field1's value to this expression - concat("00",/my:myFields/my:field1)

Rule 2: Condition - If field1 matches the pattern \d\d (this means two integers)

           Action: Set field1's value to this expression - concat("0",/my:myFields/my:field1)

 I've attached a sample form that will do this for you. I also added some data validation to the field to ensure there cannot be more that three numbers in the field.

This formula should also work for adding leading zeros to make three digits:

concat(substring('000', 1, 3 - string-length(field)), field)