I have two or three ideas of using loops to achieve data transformation for data manipulation, but I can't achieve it, ask for help, modify and improve the program to realize the use of loops to complete data transformation operations

raw data as follows:

Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input byte group str3 keys str11 contens
1 "A"   "A"          
1 "B"   "B"          
1 "str" "a"          
2 "A"   "A"          
2 "B"   "B"          
2 "str" "a b"        
3 "A"   "A"          
3 "B"   "B"          
3 "str" "a b c d"    
4 "A"   "A"          
4 "B"   "B"          
4 "str" "a b c d e f"
5 "A"   "A"          
5 "B"   "B"          
5 "str" "a b"        
end




target data as follows:


Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input byte group str3 keys str1 contents
1 "A"   "A"
1 "B"   "B"
1 "str" "a"
2 "A"   "A"
2 "B"   "B"
2 "str" "a"
2 "str" "b"
3 "A"   "A"
3 "B"   "B"
3 "str" "a"
3 "str" "b"
3 "str" "c"
3 "str" "d"
4 "A"   "A"
4 "B"   "B"
4 "str" "a"
4 "str" "b"
4 "str" "c"
4 "str" "d"
4 "str" "e"
4 "str" "f"
5 "A"   "A"
5 "B"   "B"
5 "str" "a"
5 "str" "b"
end

I got 3 ideas to solve the problem with loops, but the program is not well written, please lend a helping hand, thank you

Idea one: First use the split command to split the string by spaces, insert a line less than the number of words by 1 (because there is a line) according to the number of words, then _g1 replaces the string, _g2 replaces the next line, according to the word The number of cycles repeats until the completion

count if keys== "str"
local tol= r(N)+_N
split contents, gen(_g)
forvalues n=1(1)`tol' {
if keys== "str" {
local wc = wordcount(contents[`n'])-1
if `wc'>= 1{
insobs `wc', after(`n')
}
replace contents = _g1 if keys== "str" // This program does not need a loop, but I don't know how to deal with it
forvalues b=2/`wc' {
replace contents[`=`n'+3-`b''] = _g`b' if keys== "str" // error weights not allowed Replace contents[_n+1] contents[_n+2] contents[_n+3] with _g2 _g3 _g4... in turn until all words are filled in
}
}
}


Idea two: Use the ends function of the egen command to split the string into two parts before and after the first space and store them in separate variables, then replace the string before the space (the first word) with the original string, and then add the string before the space (the first word). Insert a line after the space, and fill in the space below the original string with the string after the space. Then the same method splits the string after the first space until it is completely filled.



count keys== "str"
local tol= r(N)+_N
forvalues n=1(1)`tol' {
if keys[`n']== "str" {
insobs 1, after(`n')
}
}

local wc = wordcount(contents[`n'])

egen contents2 = ends(contents),punct(" ")
egen contents3 = ends(contents),punct(" ") tail // Split the string in the contents variable into two parts according to the first space, and then loop
replace contents = contents2 if keys == "str"
replace contents[_n+1] = contents3[_n] if keys[_n+1] == "" // error weights not allowed
drop contents2 contents3




Idea three: Similar to idea 2, use regular expressions to match the words before the space and the words after the space in the string, store them in the temporary element, and then insert them cyclically according to the number of words. This method avoids generation and deletion. variable

local first = ustrregexs(1) if ustrregexm(contents,（"\w+") // matches the word before the first space, but I don't get the regex to match
local tail = ustrregexs(2) if ustrregexm(contents,（？) ) // matches the word after the first space


Thank you, please help me to see if my idea works? No matter what kind of solution is very helpful, how to improve the above program, I look forward to your help, For any of these ideas to improve or have a better update method, I am very grateful to you.