in reply to Molecular weight of Protein

You could also use: "Positive lookahead" 

$_ = '>gi|226694487|sp|Q1DF98.2|ATPF_MYXXD RecName: Full=ATP synthase 
+subunit b; AltName: Full=ATP synthase F(0) sector subunit b; AltName:
+ Full=ATPase subunit I; AltName: Full=F-type ATPase subunit b; Short=
+F-ATPase subunit b';

 print $1 if/(.+?)(?= RecName)/;

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NOTE: Am only showing the use of positive lookahead using the OP.

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

In Section Seekers of Perl Wisdom