StrSubs(s, pattern, replace [, n])

or

string.gsub(s, pattern, replace [, n])

This is a very powerful function and can be used many ways.  Used simply, it can replace all instances of the pattern.  A pair of values is returned with the modified string and the number of substitutions made.  The optional fourth argument n can be used to limit the number of substitutions made.

     > print(StrSubs("Hello there", "there", "HyperMath user"))

Hello HyperMath user  1

> print(StrSubs("talk and talk", "talk", "walk", 1))  // limit substitutions made to 1

walk and talk 1

The pattern capture, along with character class, can be used for more powerful substitutions.  For example, replace all characters inside braces {}.

> print(StrSubs("Replace {this}","{(.-)}", "that" ))// replace all characters inside braces

Replace {that}        1

If a capture is used, this can be referenced in the replacement string using the notation %capture_index.  For example:

> print(StrSubs("Jan 31", "(%a*) (%d*)", "%2 %1")) // swaps

31 Jan        2

In the above example, the first capture is indexed as %1 and the second one as %2.