fix: add documentation on substitution and shell variable expansion

README.md

@@ -478,6 +478,13 @@ rnr -f '(\w+)-(\d+).(\w+)' '${2}-${1}.${3}' ./*
 ├── 02-file.txt
 └── 03-file.txt
 ```
+__SHELL NOTE:__ In shells like Bash and zsh, make sure to wrap the `REPLACEMENT` pattern in single quotes. Otherwise, capture group indices will be replaced by expanded shell variables which will most likely be empty:
+```
+# Command typed in the shell
+rnr -f '(\w+)-(\d+).(\w+)' "${2}-${1}.${3}" ./*
+# After parameter expansion, the command that is actually ran:
+rnr -f '(\w+)-(\d+).(\w+)' "-." ./*
+```
 #### Capture several named groups and swap them
 1. Capture two digits as `number`.
 2. Capture extension as `ext`.

src/app.rs

@@ -88,7 +88,7 @@ pub fn create_app<'a>() -> App<'a, 'a> {
  )
  .arg(
  Arg::with_name("REPLACEMENT")
- .help("Expression replacement")
+ .help("Expression replacement. Enclose in single quotes to prevent variable expansion")
  .required(true)
  .validator_os(is_valid_string)
  .index(2),