Most of the other solutions don't consider text that spans multiple lines while still only highlighting the text, and they are all pretty hacky involving extra subviews.
An iOS 6 and later solution is to use attributed strings:
NSMutableAttributedString *s =
[[NSMutableAttributedString alloc] initWithString:yourString];
[s addAttribute:NSBackgroundColorAttributeName
value:[UIColor greenColor]
range:NSMakeRange(0, s.length)];
label.attributedText = s;
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