How do I make tcsh's history isearch (ctrl-r) case insensitive? I tried "set complete=igncase" but that affects file completion, not history isearch.
History search in tcsh always looks for an exact match (both incremental and nonincremental)¹. If you want a case-insensitive search, code it yourself, or make a feature request — but don't hold your breath, tcsh hasn't been actively developed for years.
I recommend switching to zsh, where incremental search is case-insensitive by default.
¹ As of tcsh 6.17.02. See