Revert 849e185 and (partially) f4cef9c (same for with field)

While python/cpython#130662 being discussed, lets keep previous (more
strict) formatting specification.

Author: skirpichev

mpmath/libmp/libmpf.py

@@ -1397,10 +1397,10 @@ def from_str(x, prec=0, rnd=round_fast, base=0):
     (?P<sign>[-+ ]?)
     (?P<no_neg_0>z)?
     (?P<alternate>\#)?
-    (?P<zeropad>0(?=0*[1-9]))?
-    (?P<width>[0-9]+)?
+    (?P<zeropad>0(?=[0-9]))?
+    (?P<width>0|[1-9][0-9]*)?
     (?P<thousands_separators>[,_])?
-    (?:\.(?P<precision>[0-9]+)(?P<frac_separators>[,_])?)?
+    (?:\.(?P<precision>0|[1-9][0-9]*)(?P<frac_separators>[,_])?)?
     (?P<rounding>[UDYZN])?
     (?P<type>[aAbeEfFgG%])?
 """, re.DOTALL | re.VERBOSE).fullmatch

mpmath/tests/test_format.py

@@ -54,18 +54,15 @@ def fmt_str(draw, types='fFeE', for_complex=False):
         skip_thousand_separators = True
 
     # Width
-    res += draw(st.sampled_from(['']*7 + list(map(str, range(1, 40)))
-                                + ([] if for_complex else ['0' + str(_)
-                                                           for _ in range(40)])))
+    res += draw(st.sampled_from(['']*7 + list(map(str, range(1, 40)))))
 
     # grouping character (thousand_separators)
     gchar = draw(st.sampled_from([''] + list(',_')))
     if gchar and not skip_thousand_separators:
         res += gchar
 
     # Precision
-    prec = draw(st.sampled_from(['']*7 + list(map(str, range(40)))
-                + ['0' + str(_) for _ in range(40)]))
+    prec = draw(st.sampled_from(['']*7 + list(map(str, range(40)))))
     if prec:
         res += '.' + prec
         # if sys.version_info >= (3, 14):
@@ -477,8 +474,6 @@ def test_mpf_fmt_cpython():
                  allow_infinity=True,
                  allow_subnormal=True))
 @example(fmt='.0g', x=9.995074823339339e-05)  # issue 880
-@example(fmt='.016f', x=0.1)  # issue 915
-@example(fmt='0030f', x=0.3)
 @example(fmt='0=13,f', x=1.1)  # issue 917
 @example(fmt='013,f', x=1.1)
 @example(fmt='013,.0%', x=1.1)