Why do I need to use double curly Brackets in my RegEx?

Question

I'm running a little regular expression in one of my xsl-transformations (xsl:analyze-string) and came across this effect that made me rather uncomfortable because I didn't really find any explanation...

I was searching for Non-Breaking-Spaces and En-Spaces, so I used the \p{Z} construct. According to many examples in the XSLT 2.0 Programmers Reference by Michael Kay, this should work. RegExBuddy also approves :)

Now my SaxonHE9.4N tells me

Error in regular expression: net.sf.saxon.trans.XPathException: expected ({)

After several trials and errors I simply doubled the Brackets \p{{Z}} ... and it worked!? But this time RegExBuddy disapproves!

Can someone give me an explanation of this effect? I couldn't find anything satisfying on the internet...

Thanks in advance!

Edit: I tried the same thing inside of a replace() function and the double bracket version didn't work. I had to do it with single brackets!

Answer 1

In an attribute value template, curly braces are special syntax indicating an XPath expression to be evaluated. If you want literal curly braces, you have to escape them by doubling:

An attribute value template consists of an alternating sequence of fixed parts and variable parts. A variable part consists of an XPath expression enclosed in curly brackets ({}). A fixed part may contain any characters, except that a left curly bracket must be written as {{ and a right curly bracket must be written as }}.

Note:

An expression within a variable part may contain an unescaped curly bracket within a StringLiteral XP or within a comment.

Not all attributes are AVTs, but the regex attribute of analyze-string is:

Note:

Because the regex attribute is an attribute value template, curly brackets within the regular expression must be doubled. For example, to match a sequence of one to five characters, write regex=".{{1,5}}". For regular expressions containing many curly brackets it may be more convenient to use a notation such as regex="{'[0-9]{1,5}[a-z]{3}[0-9]{1,2}'}", or to use a variable.

(Emphasis added, in both quotes.)