It's a two-stage substitution, allowing you to usevariables within the interpretation. Normally, with an expression like:

%str:~start,len%

thestart andlen must be numeric constants rather than variables. So this is the way you use a variable instead of a constant.

Starting with:

set mmm=%%mon:~%id%,3%%

In the first stage,%% markers are replaced with% and%id% is replaced with123, giving:

set mmm=%mon:~123,3%

In the second stage, it's interpreted as you would expect, although the offset123 seems a little strange in this case.

You can see the effect here:

@setlocal enableextensions enabledelayedexpansion@echo offrem               1         2         3         4rem     01234567890123456789012345678901234567890123456set mon=JAN.FEB.MAR.APR.MAY.JUN.JUL.AUG.SEP.OCT.NOV.DECrem                                 ^rem                                28set id=28call set mmm=%%mon:~%id%,3%%echo %mmm%endlocal

which outputsAUG.

Two steps:

first, the value ofid is substituted into the expression, so it becomesset mmm=%%mon:~123,3%%

Then the expression is evaluated - as a single-line "subroutine", removing one level of% :set mmm=%mon:~123,3%

so, attempts to setmmm to the 3 characters following the 1323rd character inmon (counting from "character 0")

Which should "set"mon tonothing

(ifid was3, thenmmm would be set toFeb)

• batch-file