They're described in the perlguts manpage. If you compile and run string . Here's a sample program, match . He fumbles through his red squeezey changepurse and gives the boy three extra pennies with his dollar, hoping that he might luck into the correct amount. The boy gives him back two of his own pennies and then the big shiny quarter that is his prize.
Text contains the word 'quarter'.
Text doesn't contain the word 'eighth'. 139 substitutions made. H fmbls thrgh hs rd sqzy chngprs nd gvs th by thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt. No substitution made. They're described in the perlguts manpage.