InDesign GREP: Unexpected behavior with OR operator and anchors

[SEASONS283724216wp2]

Problem Description

I've discovered that InDesign GREP behaves differently from standard regex engines when using the OR operator (|) combined with anchors (^ and $). This seems to be an undocumented behavior that differs significantly from what most regex documentation would suggest.

Test Cases

Test Case 1: ^ab|c

Text: ab ac bc Expected (standard regex): Matches "ab" at line start AND "c" anywhere Actual InDesign result: Only matches "ab" at line start

Test Case 2: ^a|bc

Text: ab ac bc Expected (standard regex): Matches "a" at line start AND "bc" anywhere Actual InDesign result: Only matches "a" at line start

Test Case 3: (^a)|bc

Text: ab ac bc Expected (standard regex): Matches "a" at line start AND "bc" anywhere Actual InDesign result: Still only matches "a" at line start

Test Case 4: (a$)|bc

Text: ab ac bca Expected (standard regex): Matches "a" at line end AND "bc" anywhere Actual InDesign result: Only matches "bc" (ignores the anchor completely)

Observations

1. InDesign GREP appears to interpret ^ab|c as ^(ab|c) instead of (^ab)|c
2. Even explicit grouping (^a)|bc doesn't work as expected
3. The $ anchor behaves inconsistently - sometimes ignored entirely
4. This behavior is not documented in Adobe's official documentation
5. Peter Kahrel's "GREP in InDesign" book notably omits any discussion of the OR operator

Questions

1. Is this a known bug or intentional design choice?
2. Are there any workarounds to achieve standard OR behavior with anchors?
3. Why does the $ anchor behave differently from ^ in OR expressions?
4. Has Adobe ever officially addressed this inconsistency?

[Scott Falkner]

I tried searching for 

^a(b|c)

and this found “ab” or “ac” at the beginning of a pragraph. Is this what you are after? 

Here I used GREP Style to highlight the affected text.

[SEASONS283724216wp2]

Thanks for the response, Scott! However, I think there might be a misunderstanding.

Your suggestion ^a(b|c) finds "ab" or "ac" at the beginning of a line, which is different from what I'm trying to achieve.

My issue is specifically with the OR operator behavior in InDesign GREP:

• ^a|bc should theoretically match "a" at line start OR "bc" anywhere in the text
• But InDesign only matches the "a" at line start and ignores "bc" completely
• Even with explicit grouping (^a)|bc, it still only matches "a"

This differs from standard regex engines where both parts of the OR expression should be matched independently.

Question: Is this a known limitation of InDesign GREP's OR operator when combined with anchors? Or is there a way to achieve true OR behavior with anchors?

The core issue is that InDesign seems to interpret ^a|bc as ^(a|bc) instead of (^a)|(bc).

[Eugene Tyson]

You’re absolutely correct, InDesign’s GREP engine does not treat the OR operator with anchors the same way standard regex does. The anchors are effectively ‘pulled out’ to cover the entire expression. A workaround is to use lookahead and lookbehind assertions to manually enforce anchoring, for example:

(^a)|(?<=.)bc

or
(^a)|bc

It is a known quirk. Adobe has never documented this fully, and most advanced InDesign GREP users learn it the hard way.

It’s not a bug as such, more of a design limitation in their simplified regex engine.

[Peter Kahrel]

> Peter Kahrel's "GREP in InDesign" book notably omits any discussion of the OR operator

Well, that's not entirely true, there is discussion with examples in various places, and there is a section on alternatives, but it's true that you wouldn't find anything on the OR operator by looking at the table of contents. So that should have been clearer, thank you for the comment. But I wasn't aware of the problems with anchors and therefore there's no mention of them.

What you describe is clearly a bug. Did you report it (at https://indesign.uservoice.com)? If you did (or will do, please post the link here so that we can vote for it.

These days I don't think you can speak of 'standard regex'. Various versions have come about over the years -- see e.g. https://www.regular-expressions.info/tools.html and Friedl's (ageing) Mastering Regular Expressions. InDesign uses Boost's regex libraries, which are widely used. It's therefore not correct to say that InDesign uses a simplified regex engine. Adobe made errors integrating the Boost libraries, that's the problem.

[SEASONS283724216wp2]

Thank you so much for confirming this is indeed a bug. Your expertise gave me the confidence to submit a comprehensive bug report to Adobe.

I also want to apologize - I was trying to mention that the OR operator isn't covered in your book's table of contents, but I may not have expressed that clearly. I've attached the bug report link below.

https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/50114952-or-operator-wi...

Thank you again for your invaluable contribution to the InDesign GREP community.

[Peter Kahrel]

Thanks for submitting the bug Report, I voted for it.