Clojure: # (hash) vs re-pattern - clojure

Question

I'm learning Clojure, and I discovered that there are two way to create a regular expression:
(re-pattern "12(ab)*34")
#"12(ab)*34"
I did a quick benchmark, and it seems that (1) is considerably faster than (2) when using boot repl (took ~7 seconds instead of ~10 seconds when in a loop).
May I ask as to why that may be? I was expecting the #"" syntax to do some clever compile-time optimisation, but instead it's slower than watching paint dry.

Answer 1

I'm not sure what you're measuring - you mentioned just construction of regex objects which is probably not what you are interested in.
However, measuring re-find to match string with regex yields quite similar results in both cases
(let [re #"12(ab)*34"
s "aanbciasdfsidufuo12ab34xcnm,xcvnm,xncv,m"]
(c/quick-bench
(re-find re s)))
Evaluation count : 907494 in 6 samples of 151249 calls.
Execution time mean : 659.120946 ns
...
(let [re (re-pattern "12(ab)*34")
s "aanbciasdfsidufuo12ab34xcnm,xcvnm,xncv,m"]
(c/quick-bench
(re-find re s)))
Evaluation count : 1018872 in 6 samples of 169812 calls.
Execution time mean : 588.138157 ns
...
There's nothing fundamentally different between literal regex syntax and re-pattern. Both end up using java.util.Pattern.compile method to compile final regex.
re-pattern source code:
...
[s] (if (instance? java.util.regex.Pattern s)
s
(. java.util.regex.Pattern (compile s))))
literal syntax is handled by LispReader$RegexReader nested class:
public static class RegexReader extends AFn{
static StringReader stringrdr = new StringReader();
public Object invoke(Object reader, Object doublequote, Object opts, Object pendingForms) {
StringBuilder sb = new StringBuilder();
...
return Pattern.compile(sb.toString());
}
}