StrongED can use the Risc OS Regular Expressions module if it is loaded. The module was ported to Risc OS by Neil Bird but is now maintained by Stefan Bellon and can be obtained from Stefan Bellon's www site. It is based upon GNU regex 0.12. Two StrongHelp manuals can be found on Steve Drain's www site, one about the RegEx module and a second on the RegEx Library, which gives more information of RegEx syntax.
The module is in !Boot.Resources.!System.310 in the ROOL distribution and should be loaded automatically: you can check by Menu on StrongED's Iconbar Icon -> Create File -> Grab Memory -> RAM Module. RegEx should be listed in the RAM Module list that opens.