string is an ensemble of routines for manipulating strings.

See Also

everything is a string
string forward compatibility
Additional string functions
string compare ...
regmap, by SS
Apply scripts to matching substrings.


official reference


string bytelength string
string cat ?string ...?
(new in Tcl 8.6.2)
string compare ?-nocase? ?-length int? string1 string2
string equal ?-nocase? ?-length int? string1 string2
string first string1 string2 ?startIndex?
string index string charIndex
string is class ?-strict? ?-failindex varname? string
string last string1 string2 ?startIndex?
string length string
string map ?-nocase? charMap string
string match ?-nocase? pattern string
string range string first last
string repeat string count
string replace string first last ?newstring?
string reverse string
string tolower string ?first? ?last?
string totitle string ?first? ?last?
string toupper string ?first? ?last?
string trim string ?chars?
string trimleft string ?chars?
string trimright string ?chars?
string wordend string charIndex
string wordstart string charIndex

Missing Functionality

pyk 2020-04-08: string is currently missing a routine, perhaps called string common, that indicates the character at which two strings diverge. Desirable options include -start and -direction.


Is this correct behavior?

% set str {} 
% string is true $str
% string is false $str
% string is integer $str
% string is alpha $str

Sadly, yes, that is correct. You'll have to use the -strict option to keep empty strings from passing all tests.

This is an unfortunate legacy from the origin of string is as a tool for entry validation, where it's important the empty string pass everything so that every input doesn't fail immediately.

Using string Functions for Binary data

In Tcl, working with binary data generally means retricting the characters in a value to the first 256 unicode characters so that the value can be neatly sent through a channel that is configured to -translation binary, where character is encoded as a single byte that has the same value as the unicode value of the character. In other words, binary data is just text that meets certain constrains.

Internally, Tcl sometimes stores binary data in a more compact form than it uses for textual data that contains a broader ranger of characters. It's not typically necesary to be aware of these internals, here is a list of routines that, as of 8.5 may take advantage of an internal ByteArray representation.

The following routines user an internal String type containing an array of unicode characters:

MG: Since Tcl 8.5, an index in the string commands can include basic math;

string range $string $startChar+1 $endChar-1

is now equivalent to

string range $string [expr {$startChar + 1}] [expr {$endChar - 1}]

While the first may be clearer, though, it seems to be (potentially quite a lot) slower for me, running 8.5a6:

% set string "This is a test string"
This is a test string
% set startChar 3
% set endChar 12
% time {string range $string [expr {$startChar+1}] [expr {$endChar-1}]} 500000
2.55498 microseconds per iteration
% time {string range $string $startChar+1 $endChar-1} 500000
5.092856 microseconds per iteration

Using expr there is quite drastically faster...