From 82b75058eaffb08f2010df99b4913c185ccfaa79 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Wed, 28 Dec 2022 20:35:23 +0530
Subject: [PATCH 01/17] performace: used regular expression 50465

---
 pandas/_libs/tslibs/parsing.pyx | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 26317da62c8d9..529cc5bdaeb4b 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -824,17 +824,11 @@ 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 = \
+        r"^\d{4}(-\d{2}(-\d{2}(T\d{2}:\d{2}:\d{2}(\.\d+)?(([+-]\d{2}:\d{2})|Z)?)?)?)?$"
     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
 
 

From 67d56121f6239e6cbc19a6d3f2f1a2d2562dca28 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Wed, 28 Dec 2022 21:34:03 +0530
Subject: [PATCH 02/17] added comments for the code

---
 pandas/_libs/tslibs/parsing.pyx | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 529cc5bdaeb4b..3c2bf9bf6eefe 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -826,7 +826,22 @@ def format_is_iso(f: str) -> bint:
     """
     iso_regex = \
         r"^\d{4}(-\d{2}(-\d{2}(T\d{2}:\d{2}:\d{2}(\.\d+)?(([+-]\d{2}:\d{2})|Z)?)?)?)?$"
+
+    # ^ -> matches the start of the string
+    # \d{4} -> matches the four digit(0-9) which represent the year
+    # -\d{2} -> matches the dash followed by 2 digits
+    # T\d{2}:\d{2}:\d{2} \
+        -> "T" followed by three groups seperated by colon: represent H:M:S
+    # (\.\d+)? -> represents the fractions of seconds
+    # (([+-]\d{2}:\d{2})|Z)? -> optional part match the time zone info
+    # ? -> The ? at the end makes the part of regex optional
+    # $ -> matches with the end
+
     excluded_formats = ["%Y%m%d", "%Y%m", "%Y"]
+
+    # uses the 're' module to check if the string matches the regular
+    # expression and not in the list of 'excluded_formats' then it will return true
+
     if re.match(iso_regex, f) and f not in excluded_formats:
         return True
     return False

From 9b13481fa82b2e98c0ec0779923476663383a8b6 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Wed, 28 Dec 2022 21:41:41 +0530
Subject: [PATCH 03/17] fixed broken comment

---
 pandas/_libs/tslibs/parsing.pyx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 3c2bf9bf6eefe..70c7aade82e2f 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -831,7 +831,7 @@ def format_is_iso(f: str) -> bint:
     # \d{4} -> matches the four digit(0-9) which represent the year
     # -\d{2} -> matches the dash followed by 2 digits
     # T\d{2}:\d{2}:\d{2} \
-        -> "T" followed by three groups seperated by colon: represent H:M:S
+    #    -> "T" followed by three groups seperated by colon: represent H:M:S
     # (\.\d+)? -> represents the fractions of seconds
     # (([+-]\d{2}:\d{2})|Z)? -> optional part match the time zone info
     # ? -> The ? at the end makes the part of regex optional

From 23679e44f18bbc6879200c36adbef75bfeb13760 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Wed, 28 Dec 2022 23:14:27 +0530
Subject: [PATCH 04/17] verbose regex

---
 pandas/_libs/tslibs/parsing.pyx | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 70c7aade82e2f..fb3254b291714 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -824,24 +824,20 @@ 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_regex = \
-        r"^\d{4}(-\d{2}(-\d{2}(T\d{2}:\d{2}:\d{2}(\.\d+)?(([+-]\d{2}:\d{2})|Z)?)?)?)?$"
-
-    # ^ -> matches the start of the string
-    # \d{4} -> matches the four digit(0-9) which represent the year
-    # -\d{2} -> matches the dash followed by 2 digits
-    # T\d{2}:\d{2}:\d{2} \
-    #    -> "T" followed by three groups seperated by colon: represent H:M:S
-    # (\.\d+)? -> represents the fractions of seconds
-    # (([+-]\d{2}:\d{2})|Z)? -> optional part match the time zone info
-    # ? -> The ? at the end makes the part of regex optional
-    # $ -> matches with the end
-
+    iso_regex = re.compile(
+        r"""
+        ^                       # start of string
+        \d{4}                   # match a 4-digit year
+        (-\d{2})?               # optionally match a 2-digit month
+        (-\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"]
-
-    # uses the 're' module to check if the string matches the regular
-    # expression and not in the list of 'excluded_formats' then it will return true
-
     if re.match(iso_regex, f) and f not in excluded_formats:
         return True
     return False

From f30c0c2efc11204f717147d932435ec6566106ed Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Thu, 29 Dec 2022 00:08:05 +0530
Subject: [PATCH 05/17] added more seperators

---
 pandas/_libs/tslibs/parsing.pyx | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index fb3254b291714..556470f6f632d 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -828,9 +828,9 @@ def format_is_iso(f: str) -> bint:
         r"""
         ^                       # start of string
         \d{4}                   # match a 4-digit year
-        (-\d{2})?               # optionally match a 2-digit month
-        (-\d{2})?               # optionally match a 2-digit day
-        (T\d{2}:\d{2}:\d{2}     # match time in the format "THH:MM:SS"
+        ([ -/\\.]\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

From 6070a00abcd8599569c53ad45b2201ed741214ee Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Thu, 29 Dec 2022 19:42:58 +0530
Subject: [PATCH 06/17] working excluding format

---
 pandas/_libs/tslibs/parsing.pyx | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 556470f6f632d..0af89b88de07f 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -818,7 +818,7 @@ class _timelex:
 _DATEUTIL_LEXER_SPLIT = _timelex.split
 
 
-def format_is_iso(f: str) -> bint:
+def format_is_iso(f: str) -> bool:
     """
     Does format match the iso8601 set that can be handled by the C parser?
     Generally of form YYYY-MM-DDTHH:MM:SS - date separator can be different
@@ -831,16 +831,23 @@ def format_is_iso(f: str) -> bint:
         ([ -/\\.]\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"
+        (\.\d+)?                # match a decimal and fractional seconds
+        ([+-]\d{2}:\d{2}|Z)?)?  # match timezone in the format "+HH:MM" or "Z"
         $                       # end of string
         """,
         re.VERBOSE,
     )
-    excluded_formats = ["%Y%m%d", "%Y%m", "%Y"]
-    if re.match(iso_regex, f) and f not in excluded_formats:
-        return True
-    return False
+
+    excluded_formats = [
+        r"^\d{4}\d{2}\d{2}$",  # %Y%m%d
+        r"^\d{4}\d{2}$",       # %Y%m
+        r"^\d{4}$",            # %Y
+    ]
+
+    if any(re.match(pattern, f) for pattern in excluded_formats):
+        return False
+
+    return bool(re.match(iso_regex, f))
 
 
 def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:

From a516dc03cbe2c2ff286d45b6b96739950a29f516 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Fri, 30 Dec 2022 18:57:12 +0530
Subject: [PATCH 07/17] test passed

---
 pandas/_libs/tslibs/strptime.pyx | 40 +++++++++++++++++++++++++-------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 27e99706137b6..133d8d42c82a9 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -14,6 +14,7 @@ from cpython.datetime cimport (
 import_datetime()
 
 from _thread import allocate_lock as _thread_allocate_lock
+import re
 
 import numpy as np
 import pytz
@@ -43,13 +44,16 @@ from pandas._libs.tslibs.np_datetime cimport (
     pydatetime_to_dt64,
     string_to_dts,
 )
+
 from pandas._libs.tslibs.np_datetime import OutOfBoundsDatetime
+
 from pandas._libs.tslibs.timestamps cimport _Timestamp
 from pandas._libs.util cimport (
     is_datetime64_object,
     is_float_object,
     is_integer_object,
 )
+
 from pandas._libs.tslibs.timestamps import Timestamp
 
 cnp.import_array()
@@ -60,15 +64,33 @@ cdef bint format_is_iso(f: str):
     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.
     """
-    excluded_formats = ["%Y%m"]
-
-    for date_sep in [" ", "/", "\\", "-", ".", ""]:
-        for time_sep in [" ", "T"]:
-            for micro_or_tz in ["", "%z", ".%f", ".%f%z"]:
-                iso_fmt = f"%Y{date_sep}%m{date_sep}%d{time_sep}%H:%M:%S{micro_or_tz}"
-                if iso_fmt.startswith(f) and f not in excluded_formats:
-                    return True
-    return False
+    iso_regex = re.compile(
+        r"""
+        ^                      # start of string
+        (?:                    # group for alternative date formats
+        %Y[ -/\\.]%m[ -/\\.]%d # 4-digit year, 2-digit month, 2-digit day
+        | %Y%m%d               # OR no separators
+        | %Y[ -/\\.]%m         # OR year & month separated by [ /, \, -]
+        | %Y%m                 # OR 4-digit year, 2-digit month, no separators
+        | %Y                   # OR 4-digit year
+        )
+        (?:                    # group for optional time and timezone
+        [ T]%H:%M:%S           # time format THH:MM:SS, T is space of T
+        (?:                    # group for optional fraction second & timezone
+        %z                     # timezone in the format +HHMM or -HHMM
+        | .%f(?:%z|Z)?         # OR format .%f%z or .%fZ
+        )?                     # make the group optional
+        )?                     # making the time and timezone grp optional
+        $                      # end of string
+        """,
+        re.VERBOSE,
+    )
+    excluded_formats = [
+        r"^%Y%m$",
+    ]
+    if any(re.match(pattern, f) for pattern in excluded_formats):
+        return False
+    return bool(re.match(iso_regex, f))
 
 
 def _test_format_is_iso(f: str) -> bool:

From 055a9207d12d646fa882c7842b2c3dafdfea57b2 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Fri, 30 Dec 2022 21:44:36 +0530
Subject: [PATCH 08/17] one seperator in a condition

---
 pandas/_libs/tslibs/strptime.pyx | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 133d8d42c82a9..86ac888d0c24b 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -68,13 +68,21 @@ cdef bint format_is_iso(f: str):
         r"""
         ^                      # start of string
         (?:                    # group for alternative date formats
-        %Y[ -/\\.]%m[ -/\\.]%d # 4-digit year, 2-digit month, 2-digit day
+        %Y[-]%m[-]%d           # year month & date separated by either hyphen
+        | %Y[/]%m[/]%d         # OR separated by forward slash
+        | %Y[.]%m[.]%d         # OR separated by period
+        | %Y[ ]%m[ ]%d         # OR separated by space
+        | %Y[\\]%m[\\]%d       # OR separated by backslash
         | %Y%m%d               # OR no separators
-        | %Y[ -/\\.]%m         # OR year & month separated by [ /, \, -]
+        | %Y[-]%m              # OR year & month separated by hyphene
+        | %Y[/]%m              # OR year & month separated by forward slash
+        | %Y[.]%m              # OR year & month separated by period
+        | %Y[ ]%m              # OR year & month separated by space
+        | %Y[\\]%m             # OR year & month separated by backslash
         | %Y%m                 # OR 4-digit year, 2-digit month, no separators
         | %Y                   # OR 4-digit year
         )
-        (?:                    # group for optional time and timezone
+       (?:                    # group for optional time and timezone
         [ T]%H:%M:%S           # time format THH:MM:SS, T is space of T
         (?:                    # group for optional fraction second & timezone
         %z                     # timezone in the format +HHMM or -HHMM

From 3f9226473bbc148a8a2731886251b93edc46488d Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Sat, 31 Dec 2022 11:02:47 +0530
Subject: [PATCH 09/17] removed unnecessary OR statements

---
 pandas/_libs/tslibs/strptime.pyx | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 86ac888d0c24b..81df27618af6c 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -74,13 +74,9 @@ cdef bint format_is_iso(f: str):
         | %Y[ ]%m[ ]%d         # OR separated by space
         | %Y[\\]%m[\\]%d       # OR separated by backslash
         | %Y%m%d               # OR no separators
-        | %Y[-]%m              # OR year & month separated by hyphene
-        | %Y[/]%m              # OR year & month separated by forward slash
-        | %Y[.]%m              # OR year & month separated by period
-        | %Y[ ]%m              # OR year & month separated by space
-        | %Y[\\]%m             # OR year & month separated by backslash
-        | %Y%m                 # OR 4-digit year, 2-digit month, no separators
-        | %Y                   # OR 4-digit year
+        | %Y[ /.\\-]%m         # OR year & month separated by [ /.\\-]
+        | %Y%m                 # OR year & month no separators
+        | %Y                   # OR just year
         )
        (?:                    # group for optional time and timezone
         [ T]%H:%M:%S           # time format THH:MM:SS, T is space of T

From 08ba66d5a97e49d735564793ea5780d949f6eb1c Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Sat, 31 Dec 2022 16:30:35 +0530
Subject: [PATCH 10/17] used backreferencing

---
 pandas/_libs/tslibs/strptime.pyx | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 81df27618af6c..ee02c2fab5901 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -68,17 +68,13 @@ cdef bint format_is_iso(f: str):
         r"""
         ^                      # start of string
         (?:                    # group for alternative date formats
-        %Y[-]%m[-]%d           # year month & date separated by either hyphen
-        | %Y[/]%m[/]%d         # OR separated by forward slash
-        | %Y[.]%m[.]%d         # OR separated by period
-        | %Y[ ]%m[ ]%d         # OR separated by space
-        | %Y[\\]%m[\\]%d       # OR separated by backslash
+        %Y([-/.\\ ])%m\1%d     # year month & date separated by either [-/.\\ ]
         | %Y%m%d               # OR no separators
         | %Y[ /.\\-]%m         # OR year & month separated by [ /.\\-]
         | %Y%m                 # OR year & month no separators
         | %Y                   # OR just year
         )
-       (?:                    # group for optional time and timezone
+        (?:                    # group for optional time and timezone
         [ T]%H:%M:%S           # time format THH:MM:SS, T is space of T
         (?:                    # group for optional fraction second & timezone
         %z                     # timezone in the format +HHMM or -HHMM

From ab0e64507886d64b342731ec7dbc270585dd9195 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Sat, 31 Dec 2022 23:41:18 +0530
Subject: [PATCH 11/17] minor bug

---
 pandas/_libs/tslibs/strptime.pyx | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index ee02c2fab5901..4278cc43a735f 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -87,6 +87,8 @@ cdef bint format_is_iso(f: str):
     )
     excluded_formats = [
         r"^%Y%m$",
+        r"^%Y[/.\\ -]%m[ |T]%H:%M.*$",
+        r"^%Y%m[ |T]%H:%M.*$",
     ]
     if any(re.match(pattern, f) for pattern in excluded_formats):
         return False

From 5561743aad06bb6554ddcf3d51eca9284830d4f2 Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Sun, 1 Jan 2023 21:00:03 +0530
Subject: [PATCH 12/17] modified version

---
 pandas/_libs/tslibs/strptime.pyx | 28 ++++++++++------------------
 1 file changed, 10 insertions(+), 18 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 4278cc43a735f..6821ec264baa9 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -66,29 +66,21 @@ cdef bint format_is_iso(f: str):
     """
     iso_regex = re.compile(
         r"""
-        ^                      # start of string
-        (?:                    # group for alternative date formats
-        %Y([-/.\\ ])%m\1%d     # year month & date separated by either [-/.\\ ]
-        | %Y%m%d               # OR no separators
-        | %Y[ /.\\-]%m         # OR year & month separated by [ /.\\-]
-        | %Y%m                 # OR year & month no separators
-        | %Y                   # OR just year
-        )
-        (?:                    # group for optional time and timezone
-        [ T]%H:%M:%S           # time format THH:MM:SS, T is space of T
-        (?:                    # group for optional fraction second & timezone
-        %z                     # timezone in the format +HHMM or -HHMM
-        | .%f(?:%z|Z)?         # OR format .%f%z or .%fZ
-        )?                     # make the group optional
-        )?                     # making the time and timezone grp optional
-        $                      # end of string
+        ^                     # start of string
+        %Y                    # Year
+        (?:([-/ \\.]?)%m      # month with or without separators
+        (?:\1%d               # day with or without separators
+        (?:[ |T]%H            # hour with separator
+        (?:\:%M               # minute with separator
+        (?:\:%S               # second with separator
+        (?:%z|.%f(?:%z|Z)?    # timezone or fractional second
+        )?)?)?)?)?)?          # optional
+        $                     # end of string
         """,
         re.VERBOSE,
     )
     excluded_formats = [
         r"^%Y%m$",
-        r"^%Y[/.\\ -]%m[ |T]%H:%M.*$",
-        r"^%Y%m[ |T]%H:%M.*$",
     ]
     if any(re.match(pattern, f) for pattern in excluded_formats):
         return False

From 42bc6997581a3853309df04a5850ac46fde1c38d Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Sun, 1 Jan 2023 21:30:18 +0530
Subject: [PATCH 13/17] non regex excluded format

---
 pandas/_libs/tslibs/strptime.pyx | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 6821ec264baa9..40ae4dd2fad38 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -44,9 +44,7 @@ from pandas._libs.tslibs.np_datetime cimport (
     pydatetime_to_dt64,
     string_to_dts,
 )
-
 from pandas._libs.tslibs.np_datetime import OutOfBoundsDatetime
-
 from pandas._libs.tslibs.timestamps cimport _Timestamp
 from pandas._libs.util cimport (
     is_datetime64_object,
@@ -69,7 +67,7 @@ cdef bint format_is_iso(f: str):
         ^                     # start of string
         %Y                    # Year
         (?:([-/ \\.]?)%m      # month with or without separators
-        (?:\1%d               # day with or without separators
+        (?: \1%d              # day with same separator as for year-month
         (?:[ |T]%H            # hour with separator
         (?:\:%M               # minute with separator
         (?:\:%S               # second with separator
@@ -79,12 +77,8 @@ cdef bint format_is_iso(f: str):
         """,
         re.VERBOSE,
     )
-    excluded_formats = [
-        r"^%Y%m$",
-    ]
-    if any(re.match(pattern, f) for pattern in excluded_formats):
-        return False
-    return bool(re.match(iso_regex, f))
+    excluded_formats = ["%Y%m"]
+    return re.match(iso_regex, f) is not None and f not in excluded_formats
 
 
 def _test_format_is_iso(f: str) -> bool:

From a22783e25ff01d043770e68ab07329044d9e96e5 Mon Sep 17 00:00:00 2001
From: Marco Edward Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Sun, 1 Jan 2023 18:05:35 +0000
Subject: [PATCH 14/17] Update pandas/_libs/tslibs/strptime.pyx

---
 pandas/_libs/tslibs/strptime.pyx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 40ae4dd2fad38..94bfa960352d8 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -71,7 +71,7 @@ cdef bint format_is_iso(f: str):
         (?:[ |T]%H            # hour with separator
         (?:\:%M               # minute with separator
         (?:\:%S               # second with separator
-        (?:%z|.%f(?:%z|Z)?    # timezone or fractional second
+        (?:%z|\.%f(?:%z|Z)?   # timezone or fractional second
         )?)?)?)?)?)?          # optional
         $                     # end of string
         """,

From 8ba5d6b0178fe76b9273e5e3ec3ea78a6867cb7e Mon Sep 17 00:00:00 2001
From: Marco Edward Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Sun, 1 Jan 2023 18:05:51 +0000
Subject: [PATCH 15/17] Update pandas/_libs/tslibs/strptime.pyx

---
 pandas/_libs/tslibs/strptime.pyx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index 94bfa960352d8..d098c34617a70 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -68,7 +68,7 @@ cdef bint format_is_iso(f: str):
         %Y                    # Year
         (?:([-/ \\.]?)%m      # month with or without separators
         (?: \1%d              # day with same separator as for year-month
-        (?:[ |T]%H            # hour with separator
+        (?:[ T]%H             # hour with separator
         (?:\:%M               # minute with separator
         (?:\:%S               # second with separator
         (?:%z|\.%f(?:%z|Z)?   # timezone or fractional second

From 287a1b9fc8263e1e25ba44b6344644c2df477fb1 Mon Sep 17 00:00:00 2001
From: Marco Edward Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Sun, 1 Jan 2023 18:08:04 +0000
Subject: [PATCH 16/17] Update pandas/_libs/tslibs/strptime.pyx

---
 pandas/_libs/tslibs/strptime.pyx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/_libs/tslibs/strptime.pyx b/pandas/_libs/tslibs/strptime.pyx
index d098c34617a70..b88a43c7c2d93 100644
--- a/pandas/_libs/tslibs/strptime.pyx
+++ b/pandas/_libs/tslibs/strptime.pyx
@@ -71,7 +71,7 @@ cdef bint format_is_iso(f: str):
         (?:[ T]%H             # hour with separator
         (?:\:%M               # minute with separator
         (?:\:%S               # second with separator
-        (?:%z|\.%f(?:%z|Z)?   # timezone or fractional second
+        (?:%z|\.%f(?:%z)?     # timezone or fractional second
         )?)?)?)?)?)?          # optional
         $                     # end of string
         """,

From f3d3b3da3a90a38e8965e871ddd48d45af2b68ae Mon Sep 17 00:00:00 2001
From: Shashwat <shashwat>
Date: Mon, 2 Jan 2023 16:58:42 +0530
Subject: [PATCH 17/17] added whatsnew

---
 doc/source/whatsnew/v2.0.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index b1387e9717079..22f6659367683 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -103,6 +103,7 @@ Other enhancements
 - :meth:`DataFrame.plot.hist` now recognizes ``xlabel`` and ``ylabel`` arguments (:issue:`49793`)
 - Improved error message in :func:`to_datetime` for non-ISO8601 formats, informing users about the position of the first error (:issue:`50361`)
 - Improved error message when trying to align :class:`DataFrame` objects (for example, in :func:`DataFrame.compare`) to clarify that "identically labelled" refers to both index and columns (:issue:`50083`)
+- Performance improvement in :func:`to_datetime` when format is given or can be inferred (:issue:`50465`)
 -
 
 .. ---------------------------------------------------------------------------