GREP of the Month: Lookahead

Many find-and-replace actions involve finding a string and replacing—or applying some formatting to—only part of the found string. But if you find a string and want to apply formatting to just part of it, you have to select the part of the string you’re interested in, do the replacement or formatting, and then find the next occurrence. All this can become very tedious very quickly. Fortunately, GREP offers a way to do conditional finds, such as “find the word Figure only if it’s followed by a digit.” These conditionals are called “lookahead,” and their general format is (?=). In our example of finding instances of the word Figure only if they’re followed by a digit, we can use the query Figure(?=sd). Note that we include in the lookahead s (the space after the word Figure) and the digit d. If you try this, you’ll see that Figure is highlighted, but the space and digits are not. This means that whatever we do now applies only to Figure. For example, if you want to italicize these instances of Figure, just set Italics in the Change Format panel. Now you can click Change All (or the more cautious Change and Change/Find) to process all remaining instances. In lookaheads, you can use other GREP constructs as well. Say you want to capture instances of the word Figure not only when they’re followed by a digit, but also by the symbol #, which you use as a placeholder, for instance. This is possible by using the character class [d#], which defines both digits and # as possible characters following Figure: Figure(?=s[d#]). If a character class is not suitable, for example, when you use a multi-character placeholder such as #@#, then you can use alternatives inside the lookahead: Figure(?=s(d|#@#)). Lookahead has a negative counterpart that

lets you match text when it’s not followed by some other text. The format of these so-called negative lookaheads is (?!). For example, to find instances of the word Figure when it’s not followed by a digit, use Figure(?!sd).