diff --git a/README.md b/README.md
index d7ae8a5..c11e82f 100644
--- a/README.md
+++ b/README.md
@@ -254,12 +254,19 @@ print(s)
 
 ### Format String
 
-A format string may be used to format an `AnsiString` or `AnsiStr` before printing. The format specification string must be in the format `"[string_format[:ansi_format]]"` where `string_format` is an extension of the standard string format specifier and `ansi_format` contains 0 or more ANSI directives separated by semicolons (;). The ANSI directives may be any of the same string values that can be passed to the `AnsiString` constructor. If no `string_format` is desired, then it can be set to an empty string.
+A format string may be used to format an `AnsiString` or `AnsiStr` before printing. The format specification string must be in the format `"[string_format[:ansi_format]]"`  where:
+- `string_format` is an extension of the standard string format specifier: `.?[+-]?[<>^]?[0-9]*`
+   - The first character is optional and is the fill character used (default: space)
+   - An optional + or - char may be specified after the first fill character to enable or disable formatting of the fill character (enabled by default)
+   - A `^`, `<`, or `>` character specified center, left, or right justification (left by default)
+   - An integer specifies the total width (0 by default)
+- `ansi_format` contains 0 or more ansi directives separated by semicolons (;). The ANSI directives may be any of the same string values that can be passed to the `AnsiString` constructor. If no `string_format` is desired, then it can be set to an empty string.
 
 Examples:
 
 ```py
 ansi_str = AnsiString("This is an ANSI string")
+
 # Right justify with width of 100, formatted with underline and colored red.
 # By default, all fill characters will take on the first character's formatting.
 print("{:>100:underline;red}".format(ansi_str))
diff --git a/src/ansi_string/ansi_string.py b/src/ansi_string/ansi_string.py
index c913d27..f9077f3 100644
--- a/src/ansi_string/ansi_string.py
+++ b/src/ansi_string/ansi_string.py
@@ -638,7 +638,7 @@ def _apply_string_format(self, string_format:str, settings:Union[AnsiFormat, Ans
             (start, end) values where accompanying formats should be applied
         '''
         extend_formatting = True
-        match = re.search(r'^(.?)([+-]?)<([0-9]*)$', string_format)
+        match = re.search(r'^(.?)([+-]?)<([0-9]*)$', string_format) or re.search(r'^()()([0-9]*)$', string_format)
         if match:
             # Left justify
             num = match.group(3)
@@ -689,11 +689,11 @@ def _apply_string_format(self, string_format:str, settings:Union[AnsiFormat, Ans
                 self.apply_formatting(settings)
             return
 
-        match = re.search(r'^[<>\^]?[+-]?[0-9]*$', string_format)
+        match = re.search(r'^[<>\^]?[+-][0-9]*$', string_format)
         if match:
             raise ValueError('Sign not allowed in string format specifier')
 
-        match = re.search(r'^[<>\^]?[ ]?[0-9]*$', string_format)
+        match = re.search(r'^[<>\^]?[ ][0-9]*$', string_format)
         if match:
             raise ValueError('Space not allowed in string format specifier')
 
@@ -731,12 +731,13 @@ def to_str(self, format_spec:str=None, optimize:bool=True, reset_start:bool=Fals
         '''
         Returns an ANSI format string with both internal and given formatting spec set.
         Parameters:
-            format_spec - must be in the format "[string_format[:ansi_format]]" where string_format is an extension of
-                          the standard string format specifier and ansi_format contains 0 or more ansi directives
-                          separated by semicolons (;)
-                          ex: ">10:bold;red" to make output right justify with width of 10, bold and red formatting
-                          No formatting should be applied as part of the justification, add a '-' after the fillchar.
-                          ex: " ->10:bold;red" to not not apply formatting to justification characters
+            format_spec - must be in the format "[string_format[:ansi_format]]" where:
+                          - string_format is an extension of the standard string format specifier: .?[+-]?[<>^]?[0-9]*
+                            An optional + or - char may be specified after the first fill character to enable or disable
+                            formatting of the fill character (enabled by default)
+                          - ansi_format contains 0 or more ansi directives separated by semicolons (;)
+                          ex: ">10:underline;red" for right justify, width of 10, underline and red formatting
+                          ex: " ->10:underline;red" to do the same but don't extend underline across fill characters
             optimize - optimization selects the shortest setting string based on the situation.
                        If this is False, then the RESET directive (0) will always used when settings change mid-string.
             reset_start - when True, the output string will always start with the RESET directive (0)
@@ -851,12 +852,13 @@ def __format__(self, __format_spec:str) -> str:
         '''
         Returns an ANSI format string with both internal and given formatting spec set.
         Parameters:
-            __format_spec - must be in the format "[string_format[:ansi_format]]" where string_format is an extension of
-                            the standard string format specifier and ansi_format contains 0 or more ansi directives
-                            separated by semicolons (;)
-                            ex: ">10:bold;red" to make output right justify with width of 10, bold and red formatting
-                            No formatting should be applied as part of the justification, add a '-' after the fillchar.
-                            ex: " ->10:bold;red" to not not apply formatting to justification characters
+            __format_spec - must be in the format "[string_format[:ansi_format]]" where:
+                          - string_format is an extension of the standard string format specifier: .?[+-]?[<>^]?[0-9]*
+                            An optional + or - char may be specified after the first fill character to enable or disable
+                            formatting of the fill character (enabled by default)
+                          - ansi_format contains 0 or more ansi directives separated by semicolons (;)
+                          ex: ">10:underline;red" for right justify, width of 10, underline and red formatting
+                          ex: " ->10:underline;red" to do the same but don't extend underline across fill characters
         '''
         return self.to_str(__format_spec)
 
diff --git a/tests/test_ansi_string.py b/tests/test_ansi_string.py
index 7556947..55042f5 100755
--- a/tests/test_ansi_string.py
+++ b/tests/test_ansi_string.py
@@ -262,6 +262,13 @@ def test_format_left_justify_and_strings(self):
             '\x1b[1;31mThis string will be formatted bold and red++++++++++++++++++++++++++++++++++++++++++++++++\x1b[m'
         )
 
+    def test_format_default_left_justify_and_strings(self):
+        s = AnsiString('This string will be formatted bold and red', 'bold')
+        self.assertEqual(
+            '{:90:fg_red}'.format(s),
+            '\x1b[1;31mThis string will be formatted bold and red                                                \x1b[m'
+        )
+
     def test_format_left_justify_no_extend_and_strings(self):
         s = AnsiString('This string will be formatted bold and red', 'bold')
         self.assertEqual(