working on documentation and more API functions

site/doc/pages/regex_replace.txt

@@ -0,0 +1,37 @@
+: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.