I am trying to extract some values from the Host field. For example, variations of host name being:
labAppdev03, labWebdev01, labDocDev02. What I am trying to get is App, Web or Doc as a role field out of host name field.
For FORMAT, it should reference the number of the capture group in your regular expression (i.e., which set of parentheses are you looking in). You only need WRITE_META for index-time field extractions, which are usually best avoided. Similarly, you can just use host here in SOURCE_KEY, since it will be a normal field at that point. Last, for your regex - either use .* to match multiple characters, or just leave it out completely since you're already matching anywhere within the hostname.