PromQL regex classes \d, \w, \s, \b match Unicode; Prometheus matches ASCII only #981

Problem, by example

Two series of x: instance="café" and instance="cafe". In an independent review probe on the branch of #974's PR, the executor's label matchers gave:

Why it happens

• Rust's regex crate makes the Perl classes \d, \w, \s and the word boundary \b Unicode-aware by default.
• Go's RE2, which Prometheus uses, defines them over ASCII only.
• The PromQL parser (promql-parser) and the executor (expressions::promql_anchored_regex) both compile patterns with Rust's regex, so the system is consistent with itself but not with Prometheus.
• (?i), the anchoring ^(?s:…)$ and Go-style literal braces (/api/{id}) already match Prometheus.

Proposal

Compile label patterns with these classes made ASCII-only, so they match Go's RE2:

• either rewrite \d, \w, \s, \b (and their negations) to (?-u:\d) and so on;
• or build the regex with regex::RegexBuilder and unicode(false) where the pattern needs no other Unicode feature.

Do this in promql_anchored_regex, the one place the executor compiles PromQL patterns. Check whether the parser's matcher, used at planning time, needs the same change.

Scope and acceptance

• Scope: executor label matchers and label_replace patterns.
• Acceptance:
  • x{instance=~"\\w+"} selects cafe but not café;
  • \d+ does not match ١٢;
  • existing matcher tests still pass.

Related

#974 (regex label matchers in the executor).