In that Coding Golf, you should convert one coding convention with TitleCase to lower_case_with_underscores. And... vice versa!

#Specification

Change the casing in a following way:

• If underscore character is a delimiter, change the casing to Title Case without any of delimiter.
• If there are multiple words with no delimiter, change the casing to lower case and add an underscore character as a delimiter.
• In case of only one word (or one character): change the casing to Title Case if the word starts with lower case; change the casing to lower case if the word starts with the upper case.

Allowed characters:

• A to Z
• a to z
• underscore (_).

Input with mixed cased words are disallowed. Examples of disallowed cases:

• Coding_Convention_Conversion
• a_BC

#Example Cases

Input                        | Expected Output
===========================================================
CodingConventionConversion   | coding_convention_conversion
coding_convention_conversion | CodingConventionConversion
abc                          | Abc
Abc                          | abc
ABC                          | a_b_c
a_b_c                        | ABC
a                            | A
A                            | a

Rules

• It is allowed to use ToUpper, ToLower and ToTitleCase functions.
• Using regular expressions is allowed.
• code-golf: shortest code in bytes wins!