collapse is not working since 1.46

[cderv]

It seems we missed a breakage in 1.46 release

With knitr 1.46

> xfun::raw_string(knitr::knit(text = c("```{r, collapse=TRUE}\n1 + 1\n1:1\n```")))
                                                                                                                                                                

``` r
1 + 1
## [1] 2
```

``` r
1:1
## [1] 1
```

With knitr 1.45

> xfun::raw_string(knitr::knit(text = c("```{r, collapse=TRUE}\n1 + 1\n1:1\n```")))
                                                                                                                                                                

```r
1 + 1
## [1] 2
1:1
## [1] 1
```

This was found at reprex side

• Code/output blocks are split tidyverse/reprex#463

[cderv]

A quick bisect shows that 5435c52 was the commit that broke

This is when we moved to xfun::fenced_block(). When there is only one class (the language), we now have a space and the regex does not handle it

Note that using several classes or attributes works as we create the other syntax

> xfun::raw_string(knitr::knit(text = c("```{r, collapse=TRUE, class.source='b'}\n1 + 1\n1:1\n```")))
                                                                                                                                                                

```{.r .b}
1 + 1
## [1] 2
1:1
## [1] 1
```

> xfun::raw_string(knitr::knit(text = c("```{r, collapse=TRUE, attr.source='b=true'}\n1 + 1\n1:1\n```")))
                                                                                                                                                                

```{.r b=true}
1 + 1
## [1] 2
1:1
## [1] 1
```

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