Question

Regular expressions can be thought of as flow diagrams. This is the diagram for the livestock expression in the previous example:

Our expression matches a string if we can find a path from the left side of the diagram to the right side. We keep a current position in the string, and every time we move through a box, we verify that the part of the string after our current position matches that box.

So if we try to match "the 3 pigs" with our regular expression, our progress through the flow chart would look like this:

• At position 4, there is a word boundary, so we can move past the first box.

• Still at position 4, we find a digit, so we can also move past the second box.

• At position 5, one path loops back to before the second (digit) box, while the other moves forward through the box that holds a single space character. There is a space here, not a digit, so we must take the second path.

• We are now at position 6 (the start of “pigs”) and at the three-way branch in the diagram. We don’t see “cow” or “chicken” here, but we do see “pig”, so we take that branch.

• At position 9, after the three-way branch, one path skips the s box and goes straight to the final word boundary, while the other path matches an s. There is an s character here, not a word boundary, so we go through the s box.

• We’re at position 10 (the end of the string) and can match only a word boundary. The end of a string counts as a word boundary, so we go through the last box and have successfully matched this string.

Conceptually, a regular expression engine looks for a match in a string as follows: it starts at the start of the string and tries a match there. In this case, there is a word boundary there, so it’d get past the first box—but there is no digit, so it’d fail at the second box. Then it moves on to the second character in the string and tries to begin a new match there... and so on, until it finds a match or reaches the end of the string and decides that there really is no match.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.