[xsl] I desire this function: substring-before(string, regex charset)

["Roger L Costello costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Folks,

The XPath substring-before function returns "that part of the given input
string that occurs before the first occurrence of the string given in $arg2."
[definition from SAXON web page]

substring-before($arg1 as xs:string?, $arg2 as xs:string?) --> xs:string

It's a shame that the value of $arg2 can't be a regex character set, e.g.,

substring-before("THEN THE CURTAIN FELL", '[AEIOU]')

returns TH.

Even better, it would be nice if there was a third argument which specified
that you also want the character that was matched from the character set:

substring-before("THEN THE CURTAIN FELL", '[AEIOU]', 'plus matched charset
character')

returns THE.

I believe such a function would be useful.

SNOBOL has such a function.

Let's see how such functionality could be used. I have this text:

THEN THE CURTAIN FELL

Fetch the string preceding the first vowel, plus the vowel:

THE

However, instead of fetching the string plus vowel, modify the text by
nullifying the string plus vowel:

N THE CURTAIN FELL

Repeat on the new, shortened text.

Here is the text as it is repeatedly shortened:

THEN THE CURTAIN FELL
N THE CURTAIN FELL
 CURTAIN FELL
RTAIN FELL
IN FELL
N FELL
LL

General Problem Statement: There is a text string. There is a character set.
Strip off the string prior to the first occurrence of a character from the
character set, plus the character. Repeat until the end of text is reached.

Below I show how to implement this in SNOBOL and then in XSLT. My XSLT
solution is large and complex. Is there a simpler, shorter solution?

First, the SNOBOL solution:

Assign the variable TEXT a string:

TEXT = "THEN THE CURTAIN FELL"

BREAK is a built-in SNOBOL function. It has one argument, which is a character
set. BREAK returns a pattern that matches a string up to but not including the
character from the character set. E.g.,

BREAK("AEIOU")

returns a pattern that matches characters up to but not including a vowel.
This pattern:

BREAK("AEIOU") LEN(1)

matches characters up to a vowel, plus the vowel.

Note: LEN(N) means, match any N-length character string. It is SNOBOL's
version of the regex .{N}

The following statement applies the pattern to TEXT, replacing the string plus
vowel with null:

TEXT BREAK("AEIOU") LEN(1) =

To incrementally strip away the string, put the statement inside a loop:

LOOP 	TEXT BREAK("AEIOU") LEN(1) =     			 :F(END)
         	OUTPUT =  TEXT					 :(LOOP)

Here is the output from running the SNOBOL program:

THEN THE CURTAIN FELL
N THE CURTAIN FELL
 CURTAIN FELL
RTAIN FELL
IN FELL
N FELL
LL

Nice.

Below is my XSLT solution. It uses the replace idea that Liam provided a few
weeks back, which is neat. Whereas the SNOBOL solution takes only 2 lines of
code, the XSLT solution requires many lines of code. Is there a simpler,
shorter solution?

Lesson Learned: when designing a new language, it might be useful for the
language to provide something like the SNOBOL BREAK function.

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
    		xmlns:xs="http://www.w3.org/2001/XMLSchema";
    		xmlns:f="function"
    		exclude-result-prefixes="#all"
    		version="3.0">

    <xsl:function name="f:remove-up-to-vowel" as="xs:string*">
        <xsl:param name="TEXT" as="xs:string"/>
        <xsl:choose>
            <!-- end of string? -->
            <xsl:when test="$TEXT eq ''"/>
            <xsl:otherwise>
                <xsl:variable name="substring-after-vowel"
 		           select="replace($TEXT, '^[^AEIOU]*[AEIOU](.*)$', '$1')"
 		           as="xs:string*"/>
                <xsl:sequence select="$substring-after-vowel"/>
                <xsl:choose>
                    <xsl:when
test="not(matches($substring-after-vowel,'[AEIOU]'))"/>
                    <xsl:otherwise>
                        <xsl:sequence
select="f:remove-up-to-vowel($substring-after-vowel)"/>
                    </xsl:otherwise>
                </xsl:choose>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:function>

    <xsl:template match="/*">
        <xsl:variable name="result"
 		   select="f:remove-up-to-vowel('THEN THE CURTAIN FELL')"
 		  as="xs:string*"/>
        <xsl:for-each select="$result">
            <xsl:message>
                <xsl:value-of select="."/>
            </xsl:message>
        </xsl:for-each>
    </xsl:template>

</xsl:stylesheet>