The specification of this function is as follows.

regexp_replace(source text, pattern text, replacement text, [flags
text])
returns text

Replace string that matches to regular expression in source text to
replacement text.

 - pattern is regular expression pattern.
 - replacement is replace string that can use '\1'-'\9', and '\&'.
    '\1'-'\9': back reference to the n'th subexpression.
    '\&'     : entire matched string.
 - flags can use the following values:
    g: global (replace all)
    i: ignore case
    When the flags is not specified, case sensitive, replace the first
    instance only.

Atsushi Ogawa