Regular expressions are used to detect and / or replace text from a given text input. RegExr is an online testing environment for formulating regular expressions — vital in checking that expressions are working as expected. There is also an Air-based desktop app available for free download.
As well as having vast useful applications for general development, the arbitrary nature of regular expressions also has significant implications for code-based language play. Loss Glazier has explored the radical potentials of the grep in Digital Poetics; the concept with regular expressions is precisely the same.
For some of Glazier’s practical applications of grep procedures, check out this selection and also the online appendices to the Digital Poetics book. See also Glazier’s paper from WITZ (Spring, 1999).