gistfile1.jsx

#target indesign;
#targetengine 'myCustomGetFileNameHandler';

if (app.activeDocument && app.activeDocument.name) {
    var underscoreIndex = app.activeDocument.name.indexOf("_");
    
    if (underscoreIndex !== -1) {
        var fileNameVariable = app.activeDocument.name.slice(0, ++underscoreIndex); // get XXXXX_ from XXXXX_Name1_Name2_Name3.indd file name
    
        function grepClearSearchFields() {
            app.findGrepPreferences = app.changeGrepPreferences = NothingEnum.nothing; // clear settings
        }

        function grepIncludeOptions() {
            app.findChangeGrepOptions.includeLockedLayersForFind = false; // search in Locked Layers
            app.findChangeGrepOptions.includeLockedStoriesForFind = false; // search in Locked Stories
            app.findChangeGrepOptions.includeHiddenLayers = false; // search in HiddenLayers
            app.findChangeGrepOptions.includeMasterPages = false; // search in Master Pages
            app.findChangeGrepOptions.includeFootnotes = true; // search in Footnotes
        }

        function grep(findWhat, changeTo) {
            grepClearSearchFields(); // clear before we start            
            grepIncludeOptions(); // load "where to search" options
            
            app.findGrepPreferences.findWhat = findWhat;
            app.changeGrepPreferences.changeTo = changeTo;
            
            var foundInstances = app.activeDocument.findGrep(); 
            
            if (foundInstances.length) {
                for(i = 0; i < foundInstances.length; i++) {
                    var foundContents = foundInstances[i].contents;

                    if (foundContents !== changeTo && 
                        confirm("Do you want to replace " + foundContents + " with " + fileNameVariable, false, "Replace?")
                    ) {
                        app.findGrepPreferences.findWhat = foundContents;
                        app.changeGrepPreferences.changeTo = changeTo;
                        
                        app.activeDocument.changeGrep();  
                    }
                }
            }
                   
            grepClearSearchFields(); // clear once we done
        } // end of GREP find / change function
    
        grep("[\\l\\u]+\\d+_", fileNameVariable);    
    } else {
        alert("Wrong file name\n" + app.activeDocument.name, "Error", 1);
    }    
} else {
    alert("No documents opened", "Error", 1);
}