Find: Two commands, one with array return, and one with boolean:

string regexFind [regex, startFrom, returnAll];

string: STRING: the string to search in
regex: STRING: regular expression
startFrom: NUMBER: the start index
returnAll: BOOL: (optional) return all results
Returns: ARRAY: Array of string-number pairs in format: [[match1, position1], [match2, position2] , ...] (if returnAll is false, only the first match is returned). If no match is found or there is an error, an empty array is returned.

string regexMatch regex

string: STRING: the string to search in
regex: STRING: regular expression
Returns: BOOL: whether a match was found

Replace: replace all found matches:

string regexReplace [regex, replace]

string: STRING: the string to search in
regex: STRING: regular expression
replace: STRING: regular expression
Returns: STRING: the string after replacing all found matches (or original string in case of error)

For single replace, one can use find and replace together:

_text = "Hello world!";
_firstMatch = _text regexFind ["\w+", 0];
_firstMatch params ["_match", "_pos"];
_newText = (_text select [0, _pos]) + (_match regexReplace ["\w+", "blabla"]) + (_text select [_pos + count _match]);