Text Processing with Ruby

Simple Parsers with StringScanner

The essence of StringScanner is in its name. It lets you scan through a string, moving through it as you capture information, always knowing where in the string you are. Unlike with a regular expression, at any point in our scanning through the string we can stop and make decisions. We can look ahead to see what’s coming next, and take a different course of action depending on the answer. We can use recursion, too, to match nested constructs.

StringScanner manages this scanning process by maintaining a scan pointer. This behaves in effectively the same way as a cursor in a text editor does, indicating what part of the string we’ve moved past and what part is yet to come. We can then use varying methods to advance ...