Regex not equal to string

I'm banging my head against a wall with a regular expression. I'm trying to define an expression that excludes exactly this text 'System' (case insensitive), but can contain the word 'System' providing it's no开发者_如何学Got just that.

Examples:

• System == INVALID
• SYSTEM == INVALID
• system == INVALID
• syStEm == INVALID
• asd SysTem == Valid
• asd System asd == Valid
• System asd == Valid
• asd System == Valid
• asd == Valid

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Try this:

^(?!system$)

Or this to match the whole line:

^(?!system$).*$

The regex has a negative look-ahead on its beginning, which doesn't match if "system" is the entire string.

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Reject if it matches ^system$ (make sure i flag is ON).

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^$|^.{1-5}$|.{7}|^[^s]|^.[^y]|^..[^s]|^...[^t]|[^e].$|[^m]$ 

But use amarghosh's answer if you can.

(updated as per suggestion below)