> For the complete documentation index, see [llms.txt](https://freebsdfrau.gitbook.io/serious-shell-programming/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://freebsdfrau.gitbook.io/serious-shell-programming/string-functions/replacestart.md).

# replacestart

Unlike Bourne Again Shell (bash), POSIX shell does not have a native find/replace function for replacing a pattern at the start of a given string with another. Below is an implementation written in native POSIX shell that is:

* Faster than piping to `awk`, `sed`, `perl`, `python`, `ruby`, or other langauge
* Compatible with `bash` and all other POSIX-compliant shells
* Optimized to use `${parameter/#pattern[/string]}` when `bash` is detected

```
 1 #!/bin/sh
 2 if [ "$BASH_VERSION" ]; then
 3     replacestart() # $var_to_get $find $replace [$var_to_set]
 4     {
 5         eval ${4:-$1}=\"\${$1/#\$2${3:+/\$3}}\"
 6     }
 7 else
 8     # NB: On FreeBSD, sh(1) runs this faster than bash(1) runs the above
 9     replacestart() # $var_to_get $find $replace [$var_to_set]
10     {
11         eval local __string=\"\$$1\" || return
12         case "$__string" in $2*)
13             __string="$3${__string#$2}"
14         esac
15         eval ${4:-$1}=\"\$__string\"
16     }
17 fi
18 eval echo \"before=[\$$1]\"
19 replacestart "$@"
20 eval echo \" after=[\$${4:-$1}]\"
```