Question

Is there a way to correctly concatenate three arbitrary XPath expressions to result in a new valid XPath expression?

Answer 1

Update: The OP has indicated in a comment that the three expressions select text nodes.

In this case using the union operator (|) seems most appropriate.

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Is there a way to correctly
concatenate three arbitrary XPath
expressions to result in a new valid
XPath expression?

There are many possible ways to do this and some of these may benefit from knowing the return type of the evaluation.

One of the combinator that always “works” (although maynot always be meaningful) is:

concat(Expr1, Expr2, Expr3)

Other examples:

number(Expr1) + number(Expr2) + number(Expr3)

boolean(Expr1) or boolean(Expr2) or boolean(Expr3)

In case the expressions are guaranteed to select a node-set, then this expression (the union of the node-sets) would also combine them:

Expr1 | Expr2 | Expr3

In XPath 2.0 this expression should always work (concatenation to a sequence of items):

Expr1 , Expr2 , Expr3

or this one:

  if(boolean(Expr1))
    then Expr2
    else Expr3