Read character 3+4 from filename and move to other position

Question

Hello,
I need a javascript to read my filename (I have the one for the full filename) and especially the 3rd and 4th position of the filename.
The characters on position 3 and 4 must be moved to the position just before the extension.
Example:
4PEN12345.pdf (old name) should be 4P12345_EN.pdf
Now I have the following script to extract all pages in an existing pdf file to separate pages with all the same suffix before the extension
/* Extract Pages to Folder */
    var re = /.*\/|\.pdf$/ig;
    var filename = this.path.replace(re,"");
        for ( var i = 0;  i < this.numPages; i++ )
        this.extractPages
            nStart: i,
            nEnd: i,
            cPath : filename + "_page_" + (i+1) + "_EN.pdf"
Can anybody help me to set me on the right direction?

Answer 1

Now I have the file 4PBG12345-1.pdf (7 pages included in the pdf).
Your script as I filled in:
/* Extract Pages to Folder */
    var re = /.*\/|\.pdf$/ig;
    var filePath = this.path.replace(this.documentFileName, "");
    var oldFileName = this.documentFileName;
    var suffix = oldFileName.substring(2,4);
    var newFileName = oldFileName.substring(0,2) + oldFileName.substring(4).replace(".pdf", "_"+suffix+".pdf");
    var newFilePath = filePath + newFileName;
        for ( var i = 0;  i < this.numPages; i++ )
        this.extractPages
            nStart: i,
            nEnd: i,
            cPath : newFilePath
Result:
1 file ==> 4P12345-1_BG.pdf (7 pages included in the pdf)
Of course, because the syntax for extracting and adding automatic pages is not filled in. So, that was my question, how must I nest to add the right:
Filepath+oldfilenamesubstring(0,2)+oldfilenamesubstring(4)+_page_i++_+suffix+.pdf in the extract section
So, my example must result in 7 pdf's:
4P12345-1_page_1_BG.pdf (with content of page 1)
4P12345-1_page_2_BG.pdf (with content of page 2)
4P12345-1_page_3_BG.pdf (with content of page 3)
4P12345-1_page_4_BG.pdf (with content of page 4)
4P12345-1_page_5_BG.pdf (with content of page 5)
4P12345-1_page_6_BG.pdf (with content of page 6)
4P12345-1_page_7_BG.pdf (with content of page 7)