You can use
<b>(?=[A-Z\s]*[a-z])(?=[a-z\s]*[A-Z])[\sA-Za-z]*(?:\(ecg\))?<\/b>
See the regex demo
Details
<b>
– a<b>
text(?=[A-Z\s]*[a-z])
– after 0 or more uppercase letters or whitespaces, there must be at least one lowercase letter(?=[a-z\s]*[A-Z])
– after 0 or more lowercase letters or whitespaces, there must be at least one uppercase letter[\sA-Za-z]*
– zero or more letters/whitespaces(?:\(ecg\))?
– an optional(ecg)
text<\/b>
– a</b>
text.
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