Good question.
The text parser documentation implies that it is using “ECMAScript (JavaScript) FLAVOR”…
Text Parser - Match Pattern (make.com)
For experimenting with regular expressions, we recommend the [regular expressions 101 website.](https://regex101.com/) Just make sure to tick the ECMAScript (JavaScript) FLAVOR in the left panel
It is also implied that replace() is using a subset of the same flavor.
String functions - replace (make.com)
"See Text parser's documentation for further information on regular expressions."
Additional note: Although the replace documentation reads that “Regular expressions (enclosed in ‘/…/’ ) can be used as search string with a combination of flags (like ‘g’ , ‘i’ , ‘m’ ) appended:”, my testing showed that ‘g’ (global) and ‘i’ (case insensitive) work, ‘m’ (multiline) and ‘s’ (single line) do not.