Merge pull request #1717 from quarto-dev/faq/rmarkdown/expr

Add a FAQ point regarding `!expr` and `!r`

Author: cderv

docs/faq/rmarkdown.qmd

@@ -99,3 +99,14 @@ You can download the latest release of RStudio from <https://posit.co/download/r
 ### Does Posit Connect support Quarto?
 
 Yes! You can publish Quarto content to Posit Connect v2021.08.0 or later. Quarto has to be enabled as documented in the Posit Connect [admin guide](https://docs.rstudio.com/connect/admin/appendix/configuration/#Quarto). Connect's user [documentation](https://docs.rstudio.com/connect/user/quarto/) refers to [Quarto.org docs](/docs/publishing/rstudio-connect.qmd#rstudio-ide) on how to publish from the RStudio IDE. To publish Python-based Quarto content, you can use the [rsconnect-python CLI](https://docs.rstudio.com/rsconnect-python/) from various locations, including VSCode, JupyterLab or the terminal.
+
+### Can I use `!expr` or `!r` syntax in YAML options and headers like I do in R Markdown?
+
+It depends on where you use them:
+
+- Inside knitr cells for options using YAML syntax: Yes, `!expr` can be used as [documented](/docs/computations/r.qmd#chunk-options). However, `!r` cannot be used because it will fail Quarto's YAML validation.
+- Inside YAML blocks for Quarto document or project configurations: No, neither `!expr` nor `!r` is supported.
+
+Quarto differs from R Markdown because it doesn't rely entirely on R for YAML processing. `!expr` is handled by the **yaml** R package, while `!r` is handled by **knitr** via `xfun::yaml_load()`. These are specific to R's YAML parsing and are not supported in Quarto's YAML metadata blocks. However, when using `engine: knitr` (the default for documents with R cells), YAML options inside knitr cells are parsed by **knitr**, allowing the use of `!expr`.
+
+Lastly, note that Quarto's YAML validation supports `!expr` but not `!r`. If you're a `!r` user, switch to `!expr` for compatibility.