Thompson was faced with a hardware environment cramped and spartan even for the time: the DEC PDP-7 on which he started in 1968 was a machine with 8K 18-bit words of memory and no software useful to him. While wanting to use a higher-level language, he wrote the original Unix system in PDP-7 assembler. At the start, he did not even program on the PDP-7 itself, but instead used a set of macros for the GEMAP assembler on a GE-635 machine. A postprocessor generated a paper tape readable by the PDP-7.
-r /REGEXP/, --regex=/REGEXP/ or --regex=@regexfile Make a tag for each line matching pattern REGEXP in the following files. regexfile is a file containing one REGEXP per line. REGEXP is anchored (as if preceded by ^). The form /REGEXP/NAME/ creates a named tag. For example Tcl named tags can be created with: --regex=/proc[ \t]+\([^ \t]+\)/\1/.