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Square Bracket protection in Tcl

I was writing some string match examples in Tcl yesterday - and had the need to explicitly match square brackets within my string. But I couldn't just write the square brackets into the match string ...

• The square bracket first needed protection from the Tcl parser before it even got to the string match command.

• Then it needed protection within the string match command to prevent it being interpreted as the start of a range.

In both cases, a \ character can be used to provide protection - that's 2 \ characters that I need - but then the \ that protects the [ in the string match itself needs protecting from the command line interpreter, making a total of three protective characters needed. Thus:

  if {[string match *\\\[* $saying]} {
    puts WOW!
  }


"Does it have to be that hard?" you ask. No - you can also use { and } to defer the string match pattern, so that it's not interpreted by the command line, thus reducing the number of backslashes needed back down to just the one:

  if {[string match {*\[*} $saying]} {
    puts WOW!
  }


Here's a longer sample program also showing the use of { and ] in a string match to indicate any one character from a range:

foreach saying {"The [world] is round"
    "pie in the sky" "Horse hooves"} {
  puts $saying
  if {[string match *\[ys\] $saying]} {
    puts Y
  }
  if {[string match {*[ys]} $saying]} {
    puts Y
  }
  if {[string match *\\\[* $saying]} {
    puts WOW!
  }
  if {[string match {*\[*} $saying]} {
    puts WOW!
  }
  }


and here is how that runs:

Dorothy:~/csr1 grahamellis$ tclsh ll2
The [world] is round
WOW!
WOW!
pie in the sky
Y
Y
Horse hooves
Y
Y
Dorothy:~/csr1 grahamellis$

(written 2007-10-23)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T205 - Tcl/Tk - String Handling in Tcl
  [404] How to check that a string contains a number in Tcl - (2005-08-06)
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1410] Tcl / regsub - changing a string and using interesting bits - (2007-10-27)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [2472] split and join in tcl and expect - (2009-10-23)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3285] Extracting data from a string / line from file - Tcl - (2011-05-10)
  [3576] Tcl - apparently odd behaviour of string trimleft - (2012-01-13)
  [4205] Regular Expression Substitution - Tcl - (2013-11-12)
  [4454] Everything is a string - even a list - (2015-03-11)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)


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Reading from another process in Tcl (pipes and sockets)
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Tcl - global, upvar and uplevel.
Square Bracket protection in Tcl
Tcl - append v lappend v concat
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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