add example using curly braces in replacement string

[lilith]

This used to work:
assert_eq!(Regex::new("(.)").unwrap().replace_all("a", "$1_"), "a_" );

But now produces "" instead of "a_".

It would appear that underscores somehow cause the replacement to be deleted instead of added.

Named captures don't fix it:

assert_eq!(Regex::new("(?P<char>.)").unwrap().replace_all("a", "$char_"), "a_" ); //actual: ""

But inserting a space between the capture and underscore works!

assert_eq!(Regex::new("(.)").unwrap().replace_all("a", "$1 _"), "a _" );
assert_eq!(Regex::new("(?P<char>.)").unwrap().replace_all("a", "$char _"), "a _" );

As does using curly braces around the identifier (the only apparent workaround)

assert_eq!(Regex::new("(?P<char>.)").unwrap().replace_all("a", "${char}_"), "a_" );

[BurntSushi]

This was an intended but undocumented breaking change. Note the docs for expand say:

name may be an integer corresponding to the index of the capture group (counted by order of opening parenthesis where 0 is the entire match) or it can be a name (consisting of letters, digits or underscores) corresponding to a named capture group.

and also

The longest possible name is used. e.g., $1a looks up the capture group named 1a and not the capture group at index 1. To exert more precise control over the name, use braces, e.g., ${1}a.

So this means that the reason why you're getting the behavior you see is that 1_ is used as the capture group name, and since that doesn't refer to any valid capture, it is replaced with the empty string. Indeed, the fix is to use curly braces.

[lilith]

Ah. I was hoping that replacements would only expand if existent in the original regex.

It'd probably be good to emphasize this, and stick to the curly brace syntax for examples. I'm not aware of other regex implementations with this behavior, thus my confusion :)

I tend to expect that identifiers don't start with numbers.

[BurntSushi]

@nathanaeljones Go's regexp engine has precisely the same semantics. But yes, better docs here would be great.