@Bruce Phillips:  Thanks for sharing your code for a trim function.

I have optimized it somewhat to hopefully:
1. Improve performance for large strings by updating the source string only after calculating the area to be trimmed.
2. Use more descriptive variable names.
3. Added option to trim left, right, or both sides of string.

But the core code is the same.

Applescript:

--

set strTest to "\tsome text here and here\t "
my trimThis(strTest, true, "full")

on trimThis(pstrSourceText, pstrCharToTrim, pstrTrimDirection)
   
   -- pstrCharToTrim     : A list of characters to trim, or true to use default
   -- pstrSourceText : The text to be trimmed
   -- pstrTrimDirection : Direction of Trim ("right","left", "full")
   
   set strTrimedText to pstrSourceText
   
   ---    USE DEFAULT IF true IS PASSED ---
   -- Lazy default (AppleScript doesn't support default values)
   
   if pstrCharToTrim is true then
       set pstrCharToTrim to {" ", tab, ASCII character 10, return, ASCII character 0}
   end if
   
   --- TRIM LEFT SIDE OF STRING ---
   
   if (pstrTrimDirection = "full") or (pstrTrimDirection = "left") then
       set iLoc to 1
       repeat until character iLoc of strTrimedText is not in pstrCharToTrim
           set iLoc to iLoc + 1
       end repeat
       
       set strTrimedText to text iLoc thru -1 of strTrimedText
   end if
   
   --- TRIM RIGHT SIDE OF STRING ---
   
   
   if (pstrTrimDirection = "full") or (pstrTrimDirection = "right") then
       set iLoc to count of strTrimedText
       repeat until character iLoc of strTrimedText is not in pstrCharToTrim
           set iLoc to iLoc - 1
       end repeat
       
       set strTrimedText to text 1 thru iLoc of strTrimedText
       
   end if
   
   return strTrimedText
   
end trimThis

And why not

Applescript:

use AppleScript version "2.4"
use framework "Foundation"

set strTest to "    Hello World! "
trimThis(strTest, missing value, left) -- result: "Hello World! "
trimThis(strTest, missing value, right) -- result: " Hello World!"
trimThis(strTest, tab & return & linefeed, missing value) -- result: "Hello World! "

on trimThis(pstrSourceText, pstrCharToTrim, pstrTrimDirection)
   -- pstrCharToTrim : A list of characters to trim, or true to use default
   -- pstrSourceText : The text to be trimmed
   -- pstrTrimDirection : Direction of Trim left, right or any value for full
   if pstrCharToTrim = missing value or pstrCharToTrim = true then
       set setToTrim to current application's NSCharacterSet's whitespaceAndNewlineCharacterSet()
   else
       set setToTrim to current application's NSCharacterSet's characterSetWithCharactersInString:pstrCharToTrim
   end if

   set anNSString to current application's NSString's stringWithString:pstrSourceText
   if pstrTrimDirection = left then
       set theRange to anNSString's rangeOfCharacterFromSet:(setToTrim's invertedSet())
       if |length| of theRange = 0 then return ""
       set anNSString to anNSString's substringFromIndex:(theRange's location)
   else if pstrTrimDirection = right then
       set theRange to anNSString's rangeOfCharacterFromSet:(setToTrim's invertedSet()) options:(current application's NSBackwardsSearch)
       if |length| of theRange = 0 then return ""
       set anNSString to anNSString's substringToIndex:(theRange's location)
   else
       set anNSString to anNSString's stringByTrimmingCharactersInSet:setToTrim
   end if
   return anNSString as text
end trimThis

Considerably slower on the sample string, but gets more competitive with longer strings.

That was no my intention and didn't meant to.

Me neither but I prefer to use Apple's guidelines and the AppleScript Language Guide who clearly says no longer to use the ASCII number and ASCII character commands since AppleScript 2.0.

With that analogy Bruce's version can still be the best, depending on the mind that's reading it

I still consider removing the possible errors and making it AppleScript 2.0 compatible as an improvement which is more than a preference. The improvement is not in performance but in reliability and support. The "right" "left" "full" was to make it cleaner (personal preference to make the if statement faster/cleaner inside the routine) and not to use true but missing value is based on the global gentlemen agreement between programmers. The latter is technically not wrong, but it's uncommon and may lead to confusing.

AS you've found, it's deprecated. And yes, it does work -- but only for true ASCII characters -- that is, for numbers 0 to 127. Above that, it's unreliable.

But there's another reason to prefer the id approach: it's much faster. The difference is insignificant in this example because you're only calling it once, but it's worth keeping in mind. Using ASCII character involves sending an Apple event, where using id does not. For example:

Applescript:

set theStart to current date
repeat 3000 times
   repeat with i from 0 to 127
       ASCII character i
   end repeat
end repeat
return (current date) - theStart

And this:

Applescript:

set theStart to current date
repeat 3000 times
   repeat with i from 0 to 127
       string id i
   end repeat
end repeat
return (current date) - theStart

Using a constant like linefeed is quicker still, but only by the narrowest of margins.