XSLT - concat & substring-after

The following example shows how to use concat and substring-after.
Concat will merge two strings and substring-after will get the string after a certain symbol.
We have two strings in the beginning:

"ABCD world"

and we want to have a "Hello world" at the end.

Here is the solution:

<xsl:value-of select="concat('Hello',substring-after('ABCD world',' '))" />