go 42 lines · 5 steps

Parsing access logs with named regex groups in Go

A precompiled regex with named capture groups turns a raw log line into a structured struct.

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Intermediate regex named-groups parsing error-handling
1package parser
2 
3import (
4 "fmt"
5 "regexp"
6)
7 
8var logPattern = regexp.MustCompile(
9 `^(?P<ip>\d{1,3}(?:\.\d{1,3}){3}) - - \[(?P<time>[^\]]+)\] "(?P<method>[A-Z]+) (?P<path>\S+) [^"]+" (?P<status>\d{3}) (?P<size>\d+)$`,
10)
11 
12type AccessEntry struct {
13 IP string
14 Time string
15 Method string
16 Path string
17 Status string
18 Size string
19}
20 
21func ParseAccessLine(line string) (*AccessEntry, error) {
22 match := logPattern.FindStringSubmatch(line)
23 if match == nil {
24 return nil, fmt.Errorf("line does not match access log format: %q", line)
25 }
26 
27 fields := make(map[string]string)
28 for i, name := range logPattern.SubexpNames() {
29 if name != "" {
30 fields[name] = match[i]
31 }
32 }
33 
34 return &AccessEntry{
35 IP: fields["ip"],
36 Time: fields["time"],
37 Method: fields["method"],
38 Path: fields["path"],
39 Status: fields["status"],
40 Size: fields["size"],
41 }, nil
42}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Compiling a regex once at package load with MustCompile avoids repeated compilation cost on every call.
  2. 2Named capture groups paired with SubexpNames let you map matches to fields without brittle positional indexing.
  3. 3Returning a nil pointer plus a descriptive error is idiomatic Go for signalling a parse that didn't match.

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