Lesson 08 · Parse a header line - Build an HTTP Server

Parse a header line

Every header is a name and a value separated by a colon. Today you split one header line into its two parts, trimming the optional whitespace HTTP allows after the colon.

The goal

Split a header line into a name and a value at the first colon, trimming surrounding whitespace from the value.

Start here - the target

TO DO

Scenario: Parsing one header field

Giventhe line "Host: example.com"

Whenit is parsed

Thenname = "Host" and value = "example.com" (the space after the colon is trimmed)

Andthe line "X-Time: 10:30" splits at the first colon only, so value = "10:30"

Background

A header field is Name: Value. The split happens at the first colon, because values are free to contain colons of their own — timestamps, URLs, and IPv6 addresses all do. Slicing at the first colon and keeping the rest verbatim avoids mangling them.

HTTP permits optional whitespace around a header value (it is called OWS in the spec), so trim spaces off the value. Leave the name exactly as written for now — the next lesson handles the fact that names are matched case-insensitively.

Make it work

i := strings.IndexByte(line, ':') // FIRST colon only
if i < 0 {
    return "", "", errors.New("bad header")
}
name := line[:i]
value := strings.TrimSpace(line[i+1:])

CheckpointDONE

One header line splits cleanly into name and value. Commit.