I have value of ArticleString in the attirbute of input xml such as

<InvoiceItem ArticleString="16.4.15.1.A.AC.F.P">

How to find 8th position either by using substring or any other soultion ?

[u]Expected output:</u> P

[u]Note: </u> Should not use last position to find P. Need to find the 8th position from the @ArticleString

[u]Example:</u> Other than using no of substring-after any other soultion available...? Just by passing postion number and return the value...?
<xsl:value-of select="substring-after(substring-after(substring-after(substring-after(@ArticleString,'.'),'.'),'.'),'.') etc..."/>

You've been asking a lot of questions here by now, so you should know that it's a good idea to say whether you want an XSLT 1.0 or 2.0 solution. (You can't expect us to remember from your previous posts.)

In 2.0, it's tokenize(@ArticleString, '\.').

In 1.0 you could use the EXSLT str:tokenize the same way.

You could write a recursive template that returns the $Nth substring by returning substring-before($arg,'.') if $N is 1, otherwise calling itself with arguments substring-after($arg, '.') and $N - 1.

Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference