string-substitute \NUM syntax with preceding backslashes

[John Foerch]

I'm seeing an odd behavior with string-substitute. I want to prefix commas in a string with backslashes, but found the following behavior:

> (use regex)

> (string-substitute #\, "\\ \\0" "a,b")
"a\\ ,b"

> (string-substitute #\, "\\\\0" "a,b")
"a\\\\0b"

The \NUM syntax in the substitution string worked as expected when preceded by a space, but not when preceded by another backslash (which was escaped).

CHICKEN version 4.9.0.1
regex version 1.0

[sjamaan]

This is an interesting bug, I would like to see what's causing this for 4.11

[sjamaan]

This seems to be an undocumented feature of the substring-replace function, which allows you to escape the backslash. I would recommend using irregex, the regex egg's API is kind of deprecated anyway, and it's also not very efficient.

Here's how to do this using irregex:

#;1> (use irregex)
#;2> (irregex-replace #\, "a,b" "\\" 0)
"a\\,b"

You can also use irregex-replace/all if you want to replace more than just the first occurance.