Date matching in file name

Sorry to ask a silly question but I just can't get Hazel to recognise date patterns in file names like these:
Oct 2015ST.pdf
Nov 2015ST.pdf
Dec 2015ST.pdf
Jan 2016ST.pdf
The pattern is <mmm><space><yyyy>ST
I want the names changed to <yyyy>-<mm> <myname.pdf>
I have tried a condition: <Name> <matches> <custom date>. The <cusotm date> is in the format <Dec><space><2015>ST
I have tried variations on that theme—none of which work.
What am I doing wrong?
Thank you in anticipation.
Oct 2015ST.pdf
Nov 2015ST.pdf
Dec 2015ST.pdf
Jan 2016ST.pdf
The pattern is <mmm><space><yyyy>ST
I want the names changed to <yyyy>-<mm> <myname.pdf>
I have tried a condition: <Name> <matches> <custom date>. The <cusotm date> is in the format <Dec><space><2015>ST
I have tried variations on that theme—none of which work.
What am I doing wrong?
Thank you in anticipation.