Argh! The m-capture regex has a bug: the final ? should be a * (and the corresponding ? in s-capture2). Naturally, this slows things down a bit (and I can't update the original answer).
Corrected code:
's-capture2' => sub { $_=$f; s/^\s*(\S+(?:\s+\S+)*)?\s*$/$1/; },
'm-capture' => sub { $_=$f; ($_) = /(\S+(?:\s+\S+)*)/; },
New results (on a different machine):
Rate Regexp-Common s-capture one-s/// s-capture2 m-c
+apture two-s///
Regexp-Common 6629/s -- -89% -90% -91%
+ -94% -97%
s-capture 62950/s 850% -- -1% -18%
+ -41% -71%
one-s/// 63621/s 860% 1% -- -17%
+ -41% -71%
s-capture2 77016/s 1062% 22% 21% --
+ -28% -65%
m-capture 107187/s 1517% 70% 68% 39%
+ -- -51%
two-s/// 219320/s 3209% 248% 245% 185%
+ 105% --
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