GREP of the Month: Positive Lookbehind

One of the most common ways to find text is based on other text that comes before or after it. For example, “only find numbers that come after a dollar symbol.”

It’s easy to find the dollar symbol and the number, you’d just use the code \$\d+ (“find a dollar symbol followed by one or more digits”). However, if you want to find “any numbers that follow a dollar symbol” (that is, the numbers but not the currency), you need to use a code based on something called the “positive lookbehind.” To get this, you first type (?<=) and then insert one or more characters after the = symbol. In this instance, the expression is (?<=\$) \d+ InDesign says “I’ll look for any series of digits, but only if it comes after a dollar sign.”

Here’s another way you can use the positive lookbehind. This code:
(?<=\d)(rd|nd|th|st)

will find st, rd, or th ordinals (such as 1st, 2nd, 3rd) that follow a digit, so that you can apply a superscript to just the ordinal and not the digit.

Note that the code after the = symbol must specify an exact number of characters. That is, you might expect the expression (?<=+) \u would mean “an uppercase letter after a string of one or more spaces.” Unfortunately, the + (“one or more”) part makes it fail because it’s too open-ended.