In Java, and it seems in a few other languages, backreferences in the pattern is preceded by a slash (e.g. \1, \2, \3, etc), but in a replacement string it's preceded by a dollar sign (e.g. $1, $2, $3, and also $0).

Here's a snippet to illustrate:

System.out.println(
    "left-right".replaceAll("(.*)-(.*)", "\\2-\\1") // WRONG!!!
); // prints "2-1"

System.out.println(
    "left-right".replaceAll("(.*)-(.*)", "$2-$1")   // CORRECT!
); // prints "right-left"

System.out.println(
    "You want million dollar?!?".replaceAll("(\\w*) dollar", "US\\$ $1")
); // prints "You want US$ million?!?"

System.out.println(
    "You want million dollar?!?".replaceAll("(\\w*) dollar", "US$ \\1")
); // throws IllegalArgumentException: Illegal group reference

Questions:

• Is the use of $ for backreferences in replacement strings unique to Java? If not, what language started it? What flavors use it and what don't?
• Why is this a good idea? Why not stick to the same pattern syntax? Wouldn't that lead to a more cohesive and an easier to learn language?
  • Wouldn't the syntax be more streamlined if statements 1 and 4 in the above were the "correct" ones instead of 2 and 3?