Getting Started with Tcl in FME: TclCaller

I've never used Tcl so far, but I recently understood its availability in FME especially on string processing. So, I decided to learn it.

I would start from the TclCaller transformer. The subject matter of examples is how to extract only upper case characters from a string attribute. This subject is from:
> RegEx for finding more than one letter in a string

Naturally, such an operation can be performed using the StringReplacer. The followings are just for getting started.

Where can we define Tcl commands for the TclCaller?
There are 3 ways to speficy Tcl commands for the TclCaller.
1. Define command(s) in "Tcl Expression" parameter
2. Define procedure(s) in "Source Code" parameter and use them in "Tcl Expression"
3. Define procedure(s) in an external file, specify the file path to "External Tcl Source File" parameter and use them in "Tcl Expression"
These are also mixable in the same TclCaller. But procedures defined in other TclCaller or Startup / Shutdown TCL Script cannot be used.

Example 1: Define commands in "Tcl Expression"
We can define commands in "Tcl Expression" directly.
FME pre-defined procedures named "FME_***" can be used here.
-----
regsub -all {[^A-Z]+} [FME_GetAttribute attr_name] {}
-----
When the attribute named "attr_name" contains "Apples Bananas Cherries", this command returns "ABC".
Tcl Commands > regsub

Example 2: Define procedures in "Source Code" or external file
In this case, we should define at least one procedure, and use it in "Tcl Expression".
FME pre-defined procedures can be used in any procedure.
-----
proc processFeature {} {
  return [regsub -all {[^A-Z]+} [FME_GetAttribute attr_name] {}]
}
-----
Tcl Expression: processFeature
-----
Tcl Commands > proc, return

The next procedure is equivalent with above, because Tcl procedure returns the value returned from the last command if there is not "return" command.
-----
proc processFeature {} {
  regsub -all {[^A-Z]+} [FME_GetAttribute attr_name] {}
}
-----

And this setting is also available.
-----
Source Code or External File: proc processFeature {s} {regsub -all {[^A-Z]+} $s {}}
-----
Tcl Expression: processFeature [FME_GetAttribute attr_name]
-----

Destination Attribute
"Tcl Expression" may return a value like the examples above, and the returned value will be saved in an attribute of the output feature. The attribute name will be the string specified as "Destination Attribute" parameter, named "_result" by default.
"Destination Attribute" parameter is not optional. If "Tcl Expression" returns no value, the destination attribute will be empty.
If the input feature had an attribute whose name is same as the destination attribute, the attribute value will be overwritten.

The TclCaller seems to always process the input features one by one. We cannot implement "group-based" processing using the TclCaller, it's a difference from the PythonCaller.

Next: AttributeCreator