C# Regex.Replace number letter number

Question

How do I replace the string "2X6X14 #2&BTR KD SPF MIRREX 1/2X5" with "2 x 6 x 14 #2&BTR KD SPF MIRREX 1/2 x 5"

Answer 1

An X with a digit either side:

var output = Regex.Replace(input, @"(\d)X(?=\d)",  "$1 x ");

This finds a digit followed by X, followed by another digit. It captures the first digit into a group $1 that is used in the replacement of eg 2X -> 2 x then moves into the next match

Or indeed as Cary commented (with the small typo of X/x), just using lookarounds:

var output = Regex.Replace(input, @"(?<=\d)X(?=\d)",  " x ");

..similar idea - finds an X between two digits, neither of which are captured or prevent matching the next X, so it's a case of replacing the found X with " x "