:sig
String regex_replace(String pattern, String replacement, String subject)
String regex_replace(String pattern, String replacement, String subject, String flags)

:params
pattern : PCRE2 regular expression pattern
replacement : replacement string
subject : string where replacements should happen
flags : optional regex flags
return value : a new string with all matches replaced

:see
>regex
regex_match
regex_search
regex_search_all
regex_split
replace

:content
Replaces every match of `pattern` in `subject` and returns the transformed string.

Example:

```uce
String html = regex_replace(
	"@([A-Za-z0-9_]+)",
	"<a href=\"/users/$1\">@$1</a>",
	"Hello @alice and @bob"
);
```

Replacement strings use PCRE2 substitution syntax, including numbered capture references such as `$1` and named references such as `${name}`.

For simple literal search-and-replace, use `replace()`. Use `regex_replace()` when the match condition needs a pattern, captures, character classes, anchors, or flags.

Invalid patterns, invalid flags, or invalid substitution syntax raise a request-visible runtime error.