Regexp语法Python

Question

这些正则表达式中的{%d}是什么？ 我读过：the re doc但我无法理解它（除了this part之外）。

B = r'( {%d})'

N = r'(\d{%d})'
NB = r'([0-9 ]{%d})'

A = r'(\w{%d})'
AB = r'([a-zA-Z ]{%d})'

AN = r'([a-zA-Z0-9]{%d})'
ANB = r'([a-zA-Z0-9 ]{%d})'

AMT = r'(\d{13}[{}A-R]{1})'

ALL = r'(.{%d})'