text: Title: [BUG-REPORT] str.replace(..., regex=True) : can't use capture group in repl str
  Body: vaex.from_arrays(s=['a,b']).s.str.replace(r'(\w+)',r'--\g<1>==',regex=True)
  
  when using capture group in <repl> str, it fails, while str_pandas.replace() is correct
  ![image](https://user-images.githubusercontent.com/87395223/144342317-58352002-1788-4a9b-a734-8da57992aca9.png)
  
  Name: vaex
  Version: 4.6.0
  Summary: Out-of-Core DataFrames to visualize and explore big tabular datasets
  Home-page: https://www.github.com/vaexio/vaex
  Author: Maarten A. Breddels
  Author-email: maartenbreddels@gmail.com
  License: MIT
  Location: /home/support/miniconda3/lib/python3.9/site-packages
  Requires: vaex-astro, vaex-core, vaex-hdf5, vaex-jupyter, vaex-ml, vaex-server, vaex-viz
  Required-by: 