used regular expression in format_is_iso()

pandas/_libs/tslibs/parsing.pyx

@@ -824,17 +824,22 @@ def format_is_iso(f: str) -> bint:
     Generally of form YYYY-MM-DDTHH:MM:SS - date separator can be different
     but must be consistent.  Leading 0s in dates and times are optional.
     """
-    iso_template = "%Y{date_sep}%m{date_sep}%d{time_sep}%H:%M:%S{micro_or_tz}".format
+    iso_regex = re.compile(
+        r"""
+        ^                       # start of string
+        \d{4}                   # match a 4-digit year
+        ([ -/\\.]\d{2}|\d{2})?  # optionally match a 2-digit month
+        ([ -/\\.]\d{2}|\d{2})?  # optionally match a 2-digit day
+        ([ T]\d{2}:\d{2}:\d{2}  # match time in the format "THH:MM:SS"
+        (\.\d+)?                # optionally match a decimal and fractional seconds
+        ([+-]\d{2}:\d{2}|Z)?)?  # optional match timezone in the format "+HH:MM" or "Z"
+        $                       # end of string
+        """,
+        re.VERBOSE,
+    )
     excluded_formats = ["%Y%m%d", "%Y%m", "%Y"]
-
-    for date_sep in [" ", "/", "\\", "-", ".", ""]:
-        for time_sep in [" ", "T"]:
-            for micro_or_tz in ["", "%z", ".%f", ".%f%z"]:
-                if (iso_template(date_sep=date_sep,
-                                 time_sep=time_sep,
-                                 micro_or_tz=micro_or_tz,
-                                 ).startswith(f) and f not in excluded_formats):
-                    return True
+    if re.match(iso_regex, f) and f not in excluded_formats:
+        return True
     return False