Update document

docs/source/user-guide/sql/scalar_functions.md

@@ -1302,13 +1302,37 @@ Apache DataFusion uses a [PCRE-like] regular expression [syntax]
 (minus support for several features including look-around and backreferences).
 The following regular expression functions are supported:
 
+- [regexp_count](#regex_count)
 - [regexp_like](#regexp_like)
 - [regexp_match](#regexp_match)
 - [regexp_replace](#regexp_replace)
 
 [pcre-like]: https://en.wikibooks.org/wiki/Regular_Expressions/Perl-Compatible_Regular_Expressions
 [syntax]: https://docs.rs/regex/latest/regex/#syntax
 
+### `regexp_count`
+Returns the number of matchs that a [regular expression] has in a string.
+
+```
+regexp_count(str, regexp[, start, flags])
+```
+
+#### Arguments
+
+- **str**: String expression to operate on.
+  Can be a constant, column, or function, and any combination of string operators.
+- **regexp**: Regular expression to test against the string expression.
+  Can be a constant, column, or function.
+- **start**: Optional start position to search for the regular expression.
+  Can be a constant, column, or function.
+- **flags**: Optional regular expression flags that control the behavior of the
+  regular expression. The following flags are supported:
+  - **i**: case-insensitive: letters match both upper and lower case
+  - **m**: multi-line mode: ^ and $ match begin/end of line
+  - **s**: allow . to match \n
+  - **R**: enables CRLF mode: when multi-line mode is enabled, \r\n is used
+  - **U**: swap the meaning of x* and x*?
+
 ### `regexp_like`
 
 Returns true if a [regular expression] has at least one match in a string,