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Regex (.NET), 74 73 bytes ^(.)* ((.)(?<=(?=\3(?<-4>.*?(?<1>\4))*)(?<-1>\1.*(?=(.)?))* .*))*$(?(1)^) Try it online! - test cases Try it on regex101 - try it on your own This uses a completely differen …

Regex (ECMAScript), 112 104 bytes ^(x+),((?=\1*.*?(((x+)(?=\5$))*)x$)(?=.*(?=\1$)(x(x(x*)\8))(?=\6*$)\3\7*$)(?=.*(?=\1*$)(\1\8+$))\9x\3)*$ Takes denominator and numerator (in that order) from input in …

Regex (ECMAScript or better), 50 bytes ^((?=(xx+?)\2+$)((?=\2+$)(?=(x+)(\4+$))\5){2,})*x$ Try it online! - ECMAScript Try it online! - Perl Try it online! - Java Try it online! - Python Try it online …

Regex (.NET), 604 517 499 469 455 bytes ^((?=[6-9](?<1>){6}|)(?=[3-59](?<1>){3}|)(?=[258](?<1>){2}|)(?=[147](?<1>)|).)+$(?<!(?=(?<-1>.)+(?(1)^))\11(?=((?<=(?=(?=((?<-2>)(?=.*$((?<-2>\2)(?<3>)|){9}).)+ …

Regex (Tcl ARE), 24 22 21 bytes ^((x((xx)*))\2*$)\3 - 19 bytes - Powers of 2 - Try it online! ^(x|(x((xx)*))\2*)$\3 - 21 bytes - Completely even - Try it online! Tcl ARE has both positive and negative …

Regex (PCRE1), 11 bytes ^(x(?1)?x|x)$ - 13 bytes - Powers of 2 ^(x(?1)?x)$ - 11 bytes - Completely even Regex engine Powers of 2 Completely even PCRE1 Try it online! Try it online! I'm not …

Regex (Java / Perl / PCRE / .NET), 12 bytes ^(\1\1|^x)*x$ - 13 bytes - Powers of 2 (\1\1|^x)+x$ - 12 bytes - Completely even Regex engine Powers of 2 Completely even Java Try it online! Try it …

Regex (.NET), 57 bytes ^(?=((?!(xx+)\2+$)(x+?))+x$)(?<-3>x){2,}(?<!\3|^\4+(x+x)) Try it online! This uses the .NET feature of balanced groups to do the prime counting, saving 3 bytes relative to the …

.NET regex, 236 229 222 210 202 198 192 bytes ^(?!(?(((?<-4>(){10})|){99}((?<4-2>)|){990}(?=[6-9](?<4>){6})?(?=[3-59](?<4>){3})?([258](?<4>)|[147])?).)+$(?(5)|(\6)?()){31}(?(4)(?(6)(?<A-6>(?<-4>))){31 …

Regex (GNU ERE or better), 12 bytes ^(.)(.*\1)?$ This regex was already used in the following answers, shown in chronological order: eush77's 12 byte GNU grep answer (GNU ERE / PCRE) Neil's edited 1 …

JavaScript (ES6), 44 bytes a=>a.reduce((b,c)=>c<25?!!b:b*(2^c<30)%72,1) Try it online! This is based on Udo Borkowski's JavaScript answer which is 49 bytes. The extra golfs are: Change (c<30?2:6) to …

Regex (ECMAScript+(?^=)RME), 34 bytes ((:x{25,}){5,})(?^1=(:.*x{30}){3}) Takes its input in unary, as a concatenation of strings of x characters whose lengths represent the numbers, separated and enc …

Regex (ECMAScript+(?^=)RME), 37 30 28 bytes (:x*)x\b(.*\1x:|(?^!.*\1\b)) Takes its input in unary, as a concatenation of strings of x characters whose lengths represent the numbers, separated and enc …

Regex (Perl / Java / PCRE2 v10.34+), 30 29 27 bytes ^(?!.*:(x(\2?+:.*\b)?){7}:) Takes its input in unary, as a concatenation of strings of x characters whose lengths represent the numbers, separated …

Regex (POSIX ERE / RE2 or better), 23 21 20 bytes ^\b((t?\But)?-tut)+$ Based on regexes by thejonymyster and Bubbler. Try it on regex101! - RE2 Try it online! - GNU ERE (Bash + egrep) Try it online! …