Regular Expressions 101

^start of string

message-id: the literal text message-id: (case insensitive)

Group 1(\S+)

\S+any non-whitespace character, repeated at least once

$end of string, or before its final line terminator

Global pattern flags

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

^message-id: (\S+)$

Message-ID: 
 <GVAP278MB0310DE2FC290B07EFC17D9BF9F989@GVAP278MB0310.CHEP278.PROD.OUTLOOK.COM>