Regular expressions

snipexp: is the one operator that does not change which sites are found - it changes what is shown for each of them. That is how a list of sites becomes a list of publisher ids, phone numbers or plugin names.

Instead of the text surrounding the match, the snippet becomes whatever your expression captures.

The expression is a PCRE pattern with its delimiters - usually |, since a query rarely contains one - and at least one capturing group, because what the group captures is what you get. Flags follow the closing delimiter, so i makes it case-insensitive. Without a capturing group the snippet comes back empty, which is the usual reason for a blank column.

snipexp:|pubid=(ra\-[\w\d]+)|i

Snippets are what the csvsnippets exports carry and what snippets=1 returns from the API. Extracting a field across a whole set of sites at once is what clusters are for. Snippets are charged against a separate daily allowance - see quotas and rate limits.