We present and illustrate Kleenex, a language for expressing general nondeterministic finite transducers, and its novel compilation to streaming string transducers with essentially optimal streaming behavior, worst-case linear-time performance and sustained high throughput. In use cases it achieves consistently high throughput rates around the 1 Gbps range on stock hardware, performing well, especially in complex use cases, in comparison to both specialized and related tools such as awk, sed, grep, RE2, Ragel and regular-expression libraries.