Data validation and custom patterns

I am trying to create a custom pattern for an InfoPath form that will follow the following format:

[IRxxxxxx] the x's are numbers and will change but I need to have the '[IR' and the closing bracket ']'.

I have a rule that if the pattern is not matched that a message comes up saying 'Use this format...[IRXXXXXX].

I have tried the following custom patterns:

[IR\d\d\d\d\d\d] - The rule does not recognize that the pattern is matched and shows the message.

[IR\d*] - Pattern not recognized and popup message appears

What is the correct syntax for the mixed text/interger entery I need?

InfoPath Office 2010