Subcommand of [string], returns the length of its argument in (Unicode) characters, which may differ from the result of [string bytelength] on the same string (because of the UTF-8 implementation).

If you want to use a name familiar from C, you might do this:
 interp alias {} strlen {} string length

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2003-10-17 in the [Tcl chatroom], some of us played around with silly pure-Tcl implementations of ''string length'':
 proc strlen s {
    set n 0
    foreach char [split $s ""] {incr n}
    set n
 } ;# RS
----
 proc strlen s {llength [split $s ""]} ;# AM
----
 proc strlen string {
    regsub {.} $string +1 string
    expr 0$string
 } ;# MS
----
 proc strlen string {expr 0[regsub -all . $string +1]} ;# dkf
----
The ''functional'' way:

 proc strlen {s} {
   expr {[regexp {.(.*)} $s - s] ? (1+[strlen $s]) : 0}
 } ;# EB

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[Category Command] | [Arts and crafts of Tcl-Tk programming]