**Feature summary**
Add functionality for gsub replacement. str_replace is currently available but does not support regexes.
**Use case(s)** (describe the actual underlying problem which you want to solve, and not only a solution):
* T27619 is stuck, but currently (on enwiki) we just add the extra variables to the filter manually, but they (or a variable containing them) needs to be copied in the filter regex for every occurrence of that letter. The expression would be more readable if we could just gsub the added_lines.
**Example:**
Currently:
```
/* Reusable letters, for when ccnorm isn't enough */
o := "ÒÔ";
exp1 := "\p[" + o + "]int"
```
After a gsub is available:
```
/* Reusable letters, for when ccnorm isn't enough */
new_added_lines := gsub(ccnorm(added_lines), "[ÒÔ]", "o");
exp1 := "\point"
```