Regular Expressions

Go's regexp package provides RE2-syntax regular expressions — compiled patterns that match strings, extract groups, and perform substitutions.

Compiling a pattern

Always compile patterns at package level with regexp.MustCompile — panics at startup if the pattern is invalid, which is the right trade-off for a literal pattern:

Use regexp.Compile (returns error) when the pattern comes from user input:

Compiled *regexp.Regexp values are safe for concurrent use.

Matching

Finding matches

Capture groups

FindAllStringSubmatch returns all matches, each as a []string.

Named groups

Replace

Split

Performance tips

• Compile once, reuse: *regexp.Regexp is goroutine-safe after compilation. Never compile inside a loop or hot path.
• Use strings for simple cases: strings.Contains, strings.HasPrefix, strings.TrimSpace are 10–100× faster than regex for fixed strings.
• POSIX vs RE2: Go's regexp package uses RE2 syntax (no lookaheads/lookbehinds) — all matches run in linear time, preventing catastrophic backtracking.