JavaScript 1.3 supports a function specification in place of the second argument of the String object's replace() method. JavaScript 1.2 supports the specification of a string only:

replace(regexp, newSubStr)

while JavaScript 1.3 supports the optional syntax:

replace(regexp, newSubFun)

The usage of a function as an argument of another function is called a "lambda expression." The function is invoked after the match is done and it dynamically generates the replacement string for the matched substring. The replace method uses the new string that the function returns as its second argument. The new string is usually determined during the function execution. Among other sources, the function can use the matched substring (determined by regexp above) to generate the new string (newSubStr above). The first parameter of the function is the complete matched substring. Other parameters can be used for matched substrings, often called "parenthetical matches" (because they are delimited with parenthesis in the regular expression).

Let's take a simple example first. Suppose we have a string that includes both a prefix and a suffix, delimited by a period. The prefix consists of any number of p's, while the suffix consists of any number of s's. The string pppp.sss is an example for such a string. The task at hand is to exchange between the prefix and the suffix. The following replace method will do it:

"pppp.sss".replace(/(p*)\.(s*)/, function (str, p1, p2)) {
                                   return p2 + "." + p1;
                                 }
                  )

The task in the following example is to convert an AM/PM time representation to a military one. The regular expression partition differentiates between the hours and the minutes in the AM/PM notation. The hour count is stored in $1 while the minute count is stored in $2, and both are passed as parameters to the computemil function.

function ampm2mil(x) {
  var s = String(x)
  var partition = /(\d\d)\:(\d\d)(\D+)\b/g;
  return s.replace(
    partition, 
    computemil ($0, $1, $2) {
      if ($2 == "AM" || $2 == "Noon") return $0 + ":" + $1
      else if  ($2 == "PM" || $2 == "Midnight") return ($0 + 12) + ":" + $1;
    } 
  )