What are the best practices for using GREP styles in long documents?

There are kinds of patterns that are known to cause slowdowns, but that's not an InDesign question as such.  I don't think I can do the topic justice, to be honest, but it's absolutely worth reading up on inefficient regular expressions. Greedy searches like .* consume more processor cycles than non-greedy searches like .*? for example. You'll find extensive discussions of the topic at places like StackExchange. But there are at least a few InDesign-specific performance issues in its GREP implementation. A positive lookbehind where you're looking for (?<=someThingBefore)mySearch requires the regex engine to run through your search more times than someThingBefore\KmySearch. 

But honestly, it's faster to move the processing work elsewhere than it is to produce quality works of efficient regular expression, by the sweat of one's brow, in order to reduce the processing load on InDesign.  One clear-cut case where regular expressions are really useful to me is when I have Arabic text with bits of parenthetical English that have to be in another font. If I am going to be expected to drop translated text from a CMS into InDesign and format it rapidly, it makes sense for me to include a GREP Style that applies that Latin-script font to any matches to [A-Za-z]+ but it doesn't make sense if I'm given a 200-page text that has supposedly been finalized and isn't likely to see much revision. It's much faster to just run a Find/Change GREP query that applies a character style to the Latin-script text, either in some text processing tool, before I place the text into InDesign, or immediately upon placing the text. (Thhat is probably the only place where I don't endorse Eugene's post in this thread wholeheartedly; I usually don't wait until the end of the document to run through a list of GREP find/change queries.)

GREP styles almost always come with performance panelties. For formatting dates, numbers, and specific patterns, create a GREP query for each and run them using a script than chains GREP queries (e.g. https://creativepro.com/files/kahrel/indesign/grep_query_runner.html).

An even simpler way is to run a script like the following. On the first line enter the names of your queries, then save the script (as a text file) and run it.

queries = ['dates', 'numbers', 'pattern1', 'pattern2'];

for (i = 0; i < queries.length; i++) {
  app.loadFindChangeQuery (queries[i], SearchModes.GREP_SEARCH);
  app.documents[0].changeGrep();
}

@johntom421 

Are you typing text from scratch in InDesign - or have you got - more or less "finished" - text from some else?

If you work on Windows - you could use free version of my tool. 

You could load all found results of the GREP query - and "accept" them on the case by case basis - but not like when you do F&C that you need to jump from one to the next result - with my tool you get a list of all results and you can tick which one one you want to change - and double-click will get you there instantly:

As powerful as GREP is, it can also cause performance issues, especially when applied across an entire document.

Further to @Doc Maik and I agree test the GREPs before applying to styles.

Instead of relying on them throughout your entire document, try using other methods like Nested Styles. Nested Styles can still work with patterns but don’t have the same processing load as GREP. If you can get the same result using Nested Styles, it’s a much lighter option for large files.

Another option is to use Find and Change manually instead of applying GREP styles in real time. This way, you can run GREP searches and apply the formatting only when you need it. A helpful trick is to run all your GREP changes at the end of the document using a GREP manager. This reduces the performance impact by not constantly checking for patterns while you’re working.

A GREP manager is a great tool to streamline this process. It allows you to batch multiple GREP searches and apply all of them in one go. That way, you aren’t constantly running these intensive checks while working, which helps keep things moving smoothly.

However, keep in mind that GREP patterns can sometimes catch things you didn’t intend, especially when dealing with dates or other ambiguous formats. For instance, a pattern that matches “3/25” could be misinterpreted as a fraction, or “26 May 2025” might end up applying a style to a sentence you didn’t want it to. It’s important to refine your GREP patterns to avoid these kinds of errors. For example, you could adjust your search to look for a number followed by a space, ensuring that only real dates get the formatting.

If your document includes different date formats or languages, you’ll need to create separate GREP patterns for each. Dates like “26 May 2025” might appear differently depending on language settings, and regional date styles can vary. So, be ready to adjust your patterns based on these variations to maintain consistency across the document.

Best to use GREP styles thoughtfully. Limit their use to where they’re truly necessary, and consider using Find and Change or a GREP manager to apply them all at once. Be mindful of the impact on performance and always test your patterns to ensure they’re working as expected. If you're dealing with different date formats or languages, fine-tune your GREP searches to avoid errors and inconsistencies.

In short, GREP is a powerful tool, but like any tool, it’s most effective when used with care. By being strategic and testing your patterns regularly, you can automate formatting without running into slowdowns or surprises.

Never done that myself, but interesting thought. For me, this would be too fuzzy, too general to auto-format certain patterns of text because it would be applied to everything the style is set for.

However, I would say: first play around with GREP styles in the Find/Change dialog, so you get the GREP. Then transport the GREP into the paragraph style and see if it does its magic as you expect or not.