ANSI CSI SGR Sequences in Rmarkdown
In fansi: ANSI Control Sequence Aware String Functions

library(fansi)
knitr::knit_hooks$set(document=function(x, options) gsub("\033", "\uFFFD", x))

Browsers Do Not Interpret ANSI CSI SGR Sequences

Over the past few years color has been gaining traction in the R terminal, particularly since Gábor Csárdi's crayon made it easy to format text with ANSI CSI SGR sequences. At the same time the advent of JJ Alaire and Yihui Xie rmarkdown and knitr packages, along with John MacFarlane pandoc, made it easy to automatically incorporate R code and output in HTML documents.

Unfortunately ANSI CSI SGR sequences are not recognized by web browsers and end up rendering weirdly¹:

sgr.string <- c(
  "\033[43;34mday > night\033[0m",
  "\033[44;33mdawn < dusk\033[0m"
)
writeLines(sgr.string)

Automatically Convert ANSI CSI SGR to HTML

fansi provides the to_html function which converts the ANSI CSI SGR sequences and OSC hyperlinks into HTML markup. When we combine it with knitr::knit_hooks we can modify the rendering of the rmarkdown document such that ANSI CSI SGR encoding is shown in the equivalent HTML.

fansi::set_knit_hooks is a convenience function that does just this. You should call it in an rmarkdown document with the:

Chunk option results set to "asis".
Chunk option comments set to "" (empty string).
The knitr::knit_hooks object as an argument.

The corresponding rmarkdown hunk should look as follows:

```r`r ''`
old.hooks <- fansi::set_knit_hooks(knitr::knit_hooks)
```

old.hooks <- fansi::set_knit_hooks(knitr::knit_hooks)

We run this function for its side effects, which cause the output to be displayed as intended:

writeLines(sgr.string)

If you are seeing extra line breaks in your output you may need to use:

```r`r ''`
old.hooks <- fansi::set_knit_hooks(knitr::knit_hooks, split.nl=TRUE)
```

If you use crayon to generate your ANSI CSI SGR style strings you may need to set options(crayon.enabled=TRUE), as in some cases crayon suppresses the SGR markup if it thinks it is not outputting to a terminal.

We can also set hooks for the other types of outputs, and add some additional CSS styles.

```r`r ''`
styles <- c(
  getOption("fansi.style", dflt_css()),  # default style
  "PRE.fansi CODE {background-color: transparent;}",
  "PRE.fansi-error {background-color: #DDAAAA;}",
  "PRE.fansi-warning {background-color: #DDDDAA;}",
  "PRE.fansi-message {background-color: #AAAADD;}"
)
old.hooks <- c(
  old.hooks,
  fansi::set_knit_hooks(
    knitr::knit_hooks,
    which=c("warning", "error", "message"),
    style=styles
) )
```

styles <- c(
  getOption("fansi.style", dflt_css()),  # default style
  "PRE.fansi CODE {background-color: transparent;}",
  "PRE.fansi-error {background-color: #DDAAAA;}",
  "PRE.fansi-warning {background-color: #DDDDAA;}",
  "PRE.fansi-message {background-color: #AAAADD;}"
)
old.hooks <- c(
  old.hooks,
  fansi::set_knit_hooks(
    knitr::knit_hooks,
    which=c("warning", "error", "message"),
    style=styles
) )

message(paste0(sgr.string, collapse="\n"))
warning(paste0(c("", sgr.string), collapse="\n"))
stop(paste0(c("", sgr.string), collapse="\n"))

You can restore the old hooks at any time in your document with:

do.call(knitr::knit_hooks$set, old.hooks)
writeLines(sgr.string)

See ?fansi::set_knit_hooks for details.

¹For illustrative purposes we output raw ANSI CSI SGR sequences in this document. However, because the ESC control character causes problems with some HTML rendering services we replace it with the � symbol. Depending on the browser and process it would normally not be visible at all, or substituted with some other symbol.