Replace text and regular expressions

Confuses me every time, so hoping to leave example this time.

Special:ReplaceText allows use of regular expressions, see their documentation.

Example:

• Page has abcdefghi
  • Original text: abcdef(.*)hi ie want it to match the "g"
  • Replacement text: wx$1yz ie want it to put the matched string between wx and yz
• results in wxgyz