Extract date from pdf and add to filename

I would like to rename several hundred pdf files containing check images downloaded from my bank (OCR'd already) by prepending the first date contained in the pdf file to the existing filename.
The pdf file contains a string "Date Posted". Immediately to the right of this is a date in the format mm/dd/yy.
All of the filenames start with the string "ck" (check). e.g. ck 2588 [description]. Assuming the Date Posted is 05/23/18, I would like to rename the file to "2018_05_23 ck 2588 [description].
Any help would be appreciated.
The pdf file contains a string "Date Posted". Immediately to the right of this is a date in the format mm/dd/yy.
All of the filenames start with the string "ck" (check). e.g. ck 2588 [description]. Assuming the Date Posted is 05/23/18, I would like to rename the file to "2018_05_23 ck 2588 [description].
Any help would be appreciated.