The split Command

The split command takes a string and turns it into a list by breaking it at specified characters and ensuring that the result has the proper list syntax. The split command provides a robust way to turn input lines into proper Tcl lists:

set line {welch:*:28405:100:Brent Welch:/usr/welch:/bin/csh}
split $line :
=> welch * 28405 100 {Brent Welch} /usr/welch /bin/csh
lindex [split $line :] 4
=> Brent Welch
					

Do not use list operations on arbitrary data.

Even if your data has space-separated words, you should be careful when using list operators on arbitrary input data. Otherwise, stray double quotes or curly braces in the input can result in invalid list structure and errors in your script. Your code will work with simple test cases, but when invalid list syntax appears in the input, your script will raise an error. The next example shows what happens when input is not a valid list. The syntax error, an unmatched quote, occurs in the middle of the list. However, you cannot access any of the list because the lindex command tries to convert the value to a list before returning any part of it.

Example 5-8 Use split to turn input data into Tcl lists.

set line {this is "not a tcl list}
lindex $line 1
=> unmatched open quote in list
lindex [split $line] 2
=> "not
					

The default separator character for split is white space, which contains spaces, tabs, and newlines. If there are multiple separator characters in a row, these result in empty list elements; the separators are not collapsed. The following command splits on commas, periods, spaces, and tabs. The backslash–space sequence is used to include a space in the set of characters. You could also group the argument to split with double quotes:

set line "	Hello, world."
split $line ,.	
=> {}Hello {}world {}
					

A trick that splits each character into a list element is to specify an empty string as the split character. This lets you get at individual characters with list operations:

split abc {}
=> a b c
					

However, if you write scripts that process data one character at a time, they may run slowly. Read Chapter 11 about regular expressions for hints on really efficient string processing.