Regex to Match HTTP Header Lines

This regex matches HTTP header lines following the format defined in RFC 7230. Here is the token-by-token breakdown:

^ — Anchors the match at the start of a line. With the m (multiline) flag, this matches at the beginning of each line, not just the beginning of the string.

([A-Za-z][A-Za-z0-9-]*) — Capturing group 1 matches the header field name. It must start with a letter followed by zero or more letters, digits, or hyphens. This covers standard headers like Content-Type, X-Custom-Header, Authorization, and Accept-Encoding.

: — Matches the literal colon that separates the header name from its value.

\s* — Matches optional whitespace after the colon. The HTTP specification allows optional whitespace between the colon and the field value.

(.+) — Capturing group 2 matches the header value. It captures one or more of any character to the end of the line. Header values can contain virtually any printable character.

$ — Anchors the match at the end of a line.

The g flag enables global matching and the m flag enables multiline mode so the anchors work per line. This is essential for parsing multi-line HTTP header blocks.

This pattern is useful for HTTP request and response parsing, debugging network traffic, building proxy servers, and analyzing log files. It correctly separates header names from values for further processing such as building header maps or validating specific headers.