:   '''[string] first''' ''string1 string2 ?startIndex?''

Search ''string2'' (the haystack) for a sequence of characters that exactly match the characters in ''string1'' (the needle). 
If found, return the index of the first character in the first such match within ''string2''. 
If not found, return “`-1`”. 
If ''startIndex'' is specified (in any of the forms accepted by [string index]), 
then the search is constrained to start with the character in ''string2'' specified by the index. 
For example,
======
string first a 0a23456789abcdef 5
======
will return “`10`”, but
======
string first a 0123456789abcdef 11
======
will return “`-1`”.

<<discussion>>

Example:
======
string first a 0a23456789abcdef
======
returns “`1`” (notice this is again a ''zero'' based value).
======
string first "a" "0a23456789abcdef" 5
======
returns “`10`”.

This example uses the ''startIndex'' to indicate that the searching should begin at the fifth character (a 4 in the example).  ''However'' the result is still a zero based index from the '''beginning''' of the string!
======
string first a 0123456789abcdef 11
======
returns a “`-1`”, the standard return if the string being sought is not found. 
An error is generated if the index is not an integer or '''end'''?'''-'''''integer''?.

----
A wish for Tcl 8.5+ or 9.0

Somehow let string first be flagged `-nocase`. 
Perhaps allowing the flag at the end where it can't be confused with and can co-exist 
with the starting index. 
Alternatively: allow [string match] to return the index at which the match occurred.
-- [Roy Terry], 7 January, 2004
<<br>>
-- [RS] same day: Isn't that duplicating what we have in
======
regexp -nocase -start 11 -inline a 0123456789abcdef 
======
Sure it is and the desire is for efficiency and also the consistency of having more string commands respect no case. There is already huge overlap between what you can do with [string] and [regexp]. 
In fact, you could surely write all the [string] commands in pure Tcl using [regexp], [regsub]. - Roy

[DKF]: In performance terms, we try to go in the ''other'' direction, rewriting [regexp] to use [string match] and [regsub] to use [string map] (well, where possible).
<<discussion>>
----
**See also**
   * [string last]
<<categories>> Tcl syntax help | Command | String Processing