Most langs use a start & end char eg '/' to determine where the match begins & ends.
For substituions, you need to use it 3 times eg
Note however, that each tool/lang has a slightly different regex engine, so consult the manual for that lang. Some langs even have their own regex engnine, plus the ability to specify using Perl-compatible regex engine (eg php does this: pcreg() ).
The definitve guide (apart from lang/tool specific books) is:
Mastering Regular Expressions
aka the 'Owl' book