Developer discovers nested capture groups silently break regex match highlighting

A developer building a personal regex tester found that live match highlighting works reliably for simple patterns but quietly fails when capture groups are nested inside one another. The tool was designed with a live RegExp builder that catches syntax errors on every keystroke, preventing crashes from propagating into the Vue reactivity system. A key discovery was that toggling the global flag changes which methods are legally callable — non-global patterns require exec(), while global ones use matchAll(), which also safely handles zero-length matches. Highlighting is rendered by walking match boundaries and rebuilding an escaped HTML string on every change, with group-level coloring only activating when the 'd' flag is manually enabled. The nested group problem stems from overlapping index ranges, which a simple left-to-right cursor walk cannot resolve without additional logic to detect and handle overlap.