Mastering Python Regular Expressions

Chapter 4. Look Around

Until this point, we have learned different mechanisms of matching characters while discarding them. A character that is already matched cannot be compared again, and the only way to match any upcoming character is by discarding it.

The exceptions to this are a number of metacharacters we have studied, the so-called zero-width assertions. These characters indicate positions rather than actual content. For instance, the caret symbol (^) is a representation of the beginning of a line or the dollar sign ($) for the end of a line. They just ensure that the position in the input is correct without actually consuming or matching any character.

A more powerful kind of zero-width assertion is look around, a mechanism with which it ...