Programming Perl, 4th Edition

Positions

Some regex constructs represent positions in the string to be matched, which is a location just to the left or right of a real character. These metasymbols are examples of zero-width assertions because they do not correspond to actual characters in the string. We often just call them “assertions”. (They’re also known as “anchors” because they tie some part of the pattern to a particular position, while the rest of the pattern is free to drift with the tide.)

You can always manipulate positions in a string without using patterns. The built-in substr function lets you extract and assign to substrings, measured from the beginning of the string, the end of the string, or from a particular numeric offset. This might be all you need if you were working with fixed-length records, for instance. Patterns are only necessary when a numeric offset isn’t sufficient. But, most of the time, substr isn’t sufficient—or at least not sufficiently convenient, compared to patterns.