The basic function for executing regular expressions is ereg. This function evaluates a string against a regular expression, returning TRUE if the pattern described by the regular expression appears in the string. In this minimal form, you can check that a string conforms to a certain form. For example, you can ensure that a U.S. postal zip code is in the proper form of five digits followed by a dash and four more digits. Listing 16.2 demonstrates this idea.
|a.c||Matches aac, abc, acc, ... — Any three-character string beginning with an a and ending with a c.|
|^a.*||Matches any string starting with an a.|
|[a-c]*x$||Matches x, ax, bx, abax, abcx — ...|