Expanding on schodckwm's reply, your regular expressions will not match as it seems you'd like them to, i.e., for:

/(I think|I believe|It seems|It\'s my opinion that|I mean|Suppose|Of course|we|they|they say)/gi

rather than matching one of:
'I think', 'I believe', 'It\'s my opinion', etc.

you will match:
'I think believet seemst\s my opinions that meanupposef courseehey say',
'I thinI believIt seemIt\s my opinion thaI meaSupposOf courswthethey say',
etc.

As schodckwm said you'll need to cluster the phrases as in:
/((I think)|(I believe)|(It seems)|(...))/gi

since you probably do not want to capture the phrases individually, but rather the outcome of the long or, you can rewrite as:
/((?:I think)|(?:I believe)|(?:It seems)|(?:...))/gi
the strings clustered with (?:string) will not be captured, so that $1 will refer to the result of the or.

-lordzoinks