SO22524822

---
title: "SO22524822"
author: "Thell"
date: "03/21/2014"
output: html_document
---

```{r setup}
# A killPrefix hook.
default_output_hook <- knitr::knit_hooks$get("output")
knitr::knit_hooks$set( output = function(x, options) {

  comment <- knitr::opts_current$get("comment")
  if( is.na(comment) ) comment <- ""
  can_null <- grepl( paste0( comment, "\\s*\\[\\d?\\]" ),
                     x, perl = TRUE)
  do_null <- isTRUE( knitr::opts_current$get("null_prefix") )
  if( can_null && do_null ) {
    # By default R print output aligns at the left brace.
    align_index <- regexpr( "\\]", x )[1] - 1
    # Two cases: start or newline
    re <- paste0( "^.{", align_index, "}\\]")
    rep <- comment
    x <- gsub( re, rep,  x )
    re <- paste0( "\\\n.{", align_index, "}\\]")
    rep <- paste0( "\n", comment )
    x <- gsub( re, rep,  x )
  }
  
  default_output_hook( x, options )
  
})

knitr::opts_template$set("kill_prefix"=list(comment=NA, null_prefix=TRUE))
```

Normal
```{r}
print( 1:50 )
```

Null prefix
```{r, null_prefix=TRUE}
print( 1:50 )
```

Set option default:
```{r}
knitr::opts_chunk$set(opts.label="kill_prefix")
```

```{r}
print( 1:50 )
```

Ensure we aren't killing strings with [:digit:] patterns.
```{r}
print( paste0( paste0("[", 1:50), "]" ),quote = FALSE)
```

Any chance of getting this to run in 2018? :-)