note emazep Inspired by BrowserUK's 2nd algorithm <strike>but slightly faster, since it calls <tt>substr</tt> just once in exchange for an extra assignment</strike>: <code> sub emazep { my $s = shift; my $tmp; ( ($tmp = substr $s, $_-1, 2) eq '11' or $tmp eq '00' ) and return for @_; return 1; } </code> Using exactly the same benchmark provided [id://349815|here] by BrowserUK, it gives: <code> Rate ysth japhy matija buk2 buk1 emazep ysth 1966/s -- -17% -87% -89% -89% -91% japhy 2361/s 20% -- -84% -87% -87% -89% matija 14719/s 648% 523% -- -19% -20% -32% buk2 18088/s 820% 666% 23% -- -1% -17% buk1 18363/s 834% 678% 25% 2% -- -16% emazep 21753/s 1006% 821% 48% 20% 18% -- </code> <br> <b>Update:</b> <br><br> added esialb's (corrected) solution to the BrowserUK's benchamrk: <code> Rate ysth japhy matija esialb buk1 buk2 emazep ysth 1975/s -- -17% -87% -88% -89% -89% -91% japhy 2374/s 20% -- -84% -85% -87% -87% -89% matija 15000/s 659% 532% -- -8% -17% -18% -30% esialb 16299/s 725% 586% 9% -- -10% -11% -23% buk1 18156/s 819% 665% 21% 11% -- -1% -15% buk2 18277/s 825% 670% 22% 12% 1% -- -14% emazep 21295/s 978% 797% 42% 31% 17% 17% -- </code> <br> <b>2nd Update:</b> <br><br> I've been misleaded by the BrowserUK solution, which contains an error which also slowed down his sub (see [id://350569|here].) After the correction his sub proves to be the fastest (dani_l's very elegant solution also included in the benchmark): <code> Rate ysth japhy esialb matija buk1 dani_l emazep buk2* ysth 1845/s -- -22% -85% -87% -88% -89% -90% -91% japhy 2366/s 28% -- -81% -84% -84% -86% -87% -88% esialb 12530/s 579% 430% -- -14% -16% -28% -31% -38% matija 14566/s 689% 516% 16% -- -2% -16% -20% -28% buk1 14885/s 707% 529% 19% 2% -- -14% -18% -27% dani_l 17357/s 841% 634% 39% 19% 17% -- -5% -14% emazep 18181/s 885% 669% 45% 25% 22% 5% -- -10% buk2* 20275/s 999% 757% 62% 39% 36% 17% 12% -- </code> <tt>buk2*</tt> is the buk2 sub as corrected by me [id://350569|here]. <br> <br> Cheers, Emanuele. 349759 349759