LibRef 4.2.1 Regular Expression Syntax 

{m,n} "...you can't omit m...."

Actually, you can: for sre, it defaults to 0

>>> a4=re.compile('a{,4}')
>>> a4.match('').group(0)
''
>>> a4.match('aaaaa').group(0)
'aaaa'

This is predictable from sre_parse.py code
            elif this == "{":
                min, max = 0, MAXREPEAT
                /* lo set to digits between { and ,.*/
                if lo: 
                    min = atoi(lo)
 
Result for pre seems buggy: compiles but does not 
match.  (so 'can't' is sort of correct, but not in way 
expected -- by raising exception):

>>> import pre
>>> pa4=pre.compile('a{,4}')
>>> pa4.match('') # None response
>>> pa4.match('aaaaa').group(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: 'NoneType' object has no 
attribute 'group'

So, suggested replacement:
"For sre, m defaults to 0; for older pre, missing m 
compiles but does not match."

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Fixed in rev. 1.95 of libre.tex; thanks!  I haven't mentioned sre vs. pre, 
because that's an implementation detail and 'pre' is likely to go away one of these versions.

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Yes: this is consistent with pre's deprecation and removal 
from re.py (after 2.2.1) and the recent removal (since 
2.2.2), from Lib Sect 4.2, of the Implementation Note 
discussing sre versus pre.