WordPress Redirection Plugin case insensitive regular expression

Let me explain what each of your attempts mean:

• (?:^|\W)someword(?:$|\W) - either the start of the string or one non-word character, followed by someword, followed by the end of the string or a non-word character
• ^/(\b\Wsomeword\W\b) - the start of the string, forward slash, followed by a word boundary, followed by a non-word character (you already have a contradiction here), followed by someword, followed by a non-word character, followed by a word boundary
• ^/(\W\bsomeword\b\W) - same with non-word characters and word boundaries switched
• ^/(?i\bsomeword\b) start of string, followed by forward slash, followed by ?!
• /(\bS|sO|oM|mE|eW|wO|oR|rD|d\b) - forward slash, followed by either a word boundary and S or sO or oM or mE or eW or wO or oR or rD or d and a word boundary

Word character - either an English letter, a digit or underscore.

Word boundary - a place where on one side you have a word character and on the other, something, which is not a word character.

(?i)\/someword$

Meaning case insensitive, there is a slash and someword, followed by the end of the string.