Help needed in writing regular expression -- TCL

Question

Hello Everyone,

Just seeking a favour to write a regular expression to match the following set of strings. I want to write an expression which matches all the following strings TCL

i) ( XYZ XZZ XVZ XWZ )

Clue : Starting string is X and Z ending string is same for all the pairs. Only the middle string is differs Y Z V W.

My trial: [regexp {^X([Y|Z|V|W]*)Z$}]

I want to write an another regexp which catches/matches only the following string wherever comes

ii) (XYZ)

My trial: [regexp {^X([Y]*)Z$}] or simply regexp {^XYZ$}

Just want to make sure its a correct approach. Is there any other way available to optimize the regexp :)

i) 1st Question Tested

set to_Match_Str "XYZ XZZ XVZ XWZ"
    foreach {wholeStr to_Match_Str} [regexp -all -inline  {X[YZVW]Z} $to_Match_Str] { 

    puts "MATCH $to_Match_Str in the list" 
    } 

It prints only XZZ XWZ from the list. Its leaves out XYZ & XVZ When I include the paranthesis [regexp -all -inline {X([YZVW])Z} $to_Match_Str]. It prints all the middle characters correctly Y Z V W

Answer 1

My trial: [regexp {^X([Y|Z|V|W]*)Z$}]

That would match the strings given, but as you are using the * multiplier it would also match strings like "XZ", "XYYYYYYYYYYYYYYYYZ" and "XYZYVWZWWWZVYYWZ". To match the middle character only once, don't use a multiplier:

^X([Y|Z|V|W])Z$

My trial: [regexp {^X([Y]*)Z$}]

The same there, it will also match strings like "XZ", "XYYZ" and "XYYYYYYYYYYYYYYYYZ". Don't put a multiplier after the set:

^X([Y])Z$

or simply regexp {^XYZ$}

That won't catch anything. To make it do the same as the other (catch the Y character), you need the parentheses:

^X(Y)Z$

Answer 2

i) (XYZ XZZ XVZ XWZ)

Clue : Starting string is X and Z ending string is same for all the pairs. Only the middle string is differs Y Z V W.

My trial: [regexp {^X([Y|Z|V|W]*)Z$}]

Assuming you're not after literal parentheses around the whole lot, you match that using this:

regexp {X([YZVW])Z} $string -> matchedSubstr

That's because the interior strings are all single characters. (It also stores the matched substring in the variable matchedSubstr; choose any variable name there that you want.) You should not use | inside a [] in a regular expression, as it has no special meaning there. (You might need to add ^$ anchors round the outside.)

On the other hand, if you want to match multiple character sequences (which the Y etc. are just stand-ins for) then you use this:

regexp {X(Y|Z|V|W)Z} $string -> matchedSubstr

Notice that | is being used here, but [] is not.

If your real string has many of these strings (whichever pattern you're using to match them) then the easiest way to extract them all is with the -all -inline options to regexp, typically used in a foreach like this:

foreach {wholeStr matchedSubstr} [regexp -all -inline {X([YZVW])Z} $string] {
    puts "Hey! I found a $matchSubstr in there!"
}

Mix and match to taste.

My trial: [regexp {^X([Y]*)Z$}] or simply regexp {^XYZ$}

Just want to make sure its a correct approach. Is there any other way available to optimize the regexp :)

That's optimal for an exact comparison. And in fact Tcl will optimize that internally to a straight string equality test if that's literal.

Answer 3

You can use the Visual Regexp tool to help, it provides feedback as you construct your regular expression.