A regex (JS RegExp source, no delimiters, e.g. "pixiv\\.net") matched case-insensitively against a full candidate URL to decide whether this plugin should handle it — the precise trigger condition for a real download/metadata fetch (the Upload page's own URL-queue grouping, the host's own find_matching_plugin). NOT what a domain-ownership lookup should match against — see PluginInfoResult.domain_match for that. Omit if this plugin has no meaningful URL-based routing (e.g. a script/login plugin, or a metadata/download plugin with no single well-known source domain).