plain strings (207) foo
anchors (208) k$
ranges (202) ^[a-f]*$
backrefs (201) (...).*\1
abba (190) ^((?!(.)(.)\3\2).)*$
a man, a plan (177) ^(.)[^p].*\1$
prime (286) ^(?!(xx+)\1+$)
four (199) (.)(.\1){3}
order (199) ^.{5}[^e]?$
triples (574) ^(([147]4|40|3[269]|9[05]|[378]1).+|0[369]*|[81][257])*$
glob (384) (rr|ll|[lbr]o|en|ta|y|cr|eat|up).*\1
balance (251) ^((<>)*|<(?!>).*>)$ //five false positives.
powers (59) ^(.|(?!.{48}$)(..)*)$ //three false positives
longcount (218) 0..1..1.. (..0){3}..1* ..0..1..1..0.. ..11(.{9}1){2}
longcount2 (218) 0..1..1.. (..0){3}..1* ..0..1..1..0.. ..11(.{9}1){2}
alphabetical (180) ^(a[er]\w+ ?)*(ass\w+ ?)*(ast+ ?)*(e\w+ ?)*(n\w+ ?)*(r\w+ ?)*(s\w+ ?)*(t\w+ ?)*$ // 4 errors
These are regex.alf.nu solutions by Bisqwit. Usernames indicated are from Reddit unless otherwise noted.
Alternatives:
ranges (202) [a-f]{4} //made of abuse.
order (199) ^[^o]?.{5}$ //another way of writing the same thing.
powers (59) ^(x|(xx){1,4}|((((((x{16})\8?)\7?)\6?)\5?)\4?)\3?)$ //zero false positives,
//with thanks to Hrafnahnef.
alphabetical (156) ^(a[er]\w+ ?)*(asse[rn]\w+ ?)*(asse[st]\w+ ?)*(ast\w+ ?)*(e[an]\w+ ?)*(e[rts]\w+ ?)*(n\w+ ?)*((?:ra|re[anr])\w+ ?)*(rese\w+ ?)*(rest\w+ ?)*(ret\w+ ?)*(se\w+ ?)*(s[nt]\w+ ?)*(t\w+ ?)*$ // no false positives
Better solutions by others:
abba (193) ^(?!.*(.)(.)\2\1) // by Laugarhraun.
triples (596) 00($|3|6|9|12|15)|4.2|.1.+4|55|.17 // by alexandrosm @ [1]
glob (397) ai|c$|^p|[bcnrw][bnopr] // by nwellnhof @ [2]
balance (287) ^(<(<(<(<(<(<.*)*>)*>)*>)*>)*>)*$ // by jensweh.
balance (288) ^(<(<(<(..)*>)*>)*>)*$ // by romanandreev @ [1]; some false positives
powers (80) ^(((x|x{8}|x{128})\3?)\2?)\1?$ // by pondscum; no false positives.
powers (97) ^(?!(.(..)+)\1*$) // by plby @ [1]; doesn't "cheat"
longcount (253) ^((.+)0 \2+1 ?)*$ // possibly by bbarry @ [3]
longcount2 (253) ^((.+)0 \2+1 ?)*$ // possibly by bbarry @ [3]
alphabetical (303) r sn|( t\w+)\1|(tat|r). r|a t| ae|e e // by alexandrosm @ [1]
Best solutions that work in the spirit of the test without abusing the test-cases:
(Where different from what is posted above; recursion shortcomings are ignored)
a man, a plan (176) ^(.)(.).*\2\1$
triples (523) ^([0369]|[258][0369]*[147]|([258][0369]*[258]|[147])([0369]|[147][0369]*[258])*([258]|[147][0369]*[147]))*$ // by Bisqwit.
balance (286) ^(<(<(<(<(<(<.*>)*>)*>)*>)*>)*>)*$ // by Overv.
order (156) ^a*b*c*d*e*f*g*h*i*j*k*l*m*n*o*p*q*r*s*t*u*v*w*x*y*z*$ // by Athox.
glob (323) ^(?:(.+) .+ \1|(.*)\*(.*) .+ \2.+\3|(.*)\*(.*)\*(.*) .+ \4.+\5.+\6|\*(.*)\*(.*)\* .+ .+\7.+\8.+)$ // by Lozzer2
glob (333) ^(\*?)(\w*)(\*?)(\w*)(\*?)(\w*) .* ((.(?!\1))+|\1)\2((.(?!\3))+|\3)\4((.(?!\5))+|\5)\6$ // by hadrel @ [2]
[1]: https://gist.github.com/jonathanmorley/8058871
[2]: https://news.ycombinator.com/item?id=6941231
[3]: https://gist.github.com/jpsim/8057500