How to find last occurrence of a substring in a given string?-CodePudding

I have a string, which describe some word, I must change ending of it to "sd", if ending == "jk". For an example, I have word: "lazerjk", I need to get from it "lazersd".

I tried to use method .gsub!, but it doesn't work correctly if we have more than one occurrence of substring "jk" in a word.

CodePudding user response：

String#rindex returns the index of the last occurrence of the given substring

String#[]= can take two integers arguments, first is index where start to replace and second - length of replaced string

You can use them this way:

replaced = "foo"
replacing = "booo"

string = "foo bar foo baz"
string[string.rindex(replaced), replaced.size] = replacing

string
# => "foo bar booo baz"

CodePudding user response：

"jughjkjkjk\njk".sub(/jk$\z/, 'sd')
 => "jughjkjkjk\nsd"

without $ is probably sufficient.

CodePudding user response：

It sounds like you're looking to replace a specific suffix only. If so, I would probably suggest using sub along with an anchored regex (to check for the desired characters only at the end of the string):

string_1 = "lazerjk"
string_2 = "lazerjk\njk"
string_3 = "lazerjkr"

string_1.sub(/jk\z/, "sd")  
#=>  "lazersd"

string_2.sub(/jk\z/, "sd")  
#=>  "lazerjk\nsd"

string_3.sub(/jk\z/, "sd")
#=>  "lazerjkr"

Or, you could do without a regex at all by using the reverse! method along with a simple conditional statement to sub! only when the suffix is present:

string = "lazerjk"
old_suffix = "jk"
new_suffix = "sd"
string.reverse!.sub!(old_suffix.reverse, new_suffix.reverse).reverse! if string.end_with? (old_suffix)
string 
#=>  "lazersd"

OR, you could even use a completely different approach. Here's an example using chomp to remove the unwanted suffix and then ljust to pad the desired suffix to the modified string.

string = "lazerjk"
string.chomp("jk").ljust(string.length, "sd")
#=>  "lazersd"

Note that the new suffix only gets added if the length of the string was modified with the initial chomp. Otherwise, the string remains unchanged.

CodePudding user response：

If the goal is to substitute the LAST OCCURRENCE (as opposed to suffix only), then this could be accomplished by using sub along with reverse:

string = "jklazerjkm"
old_substring = "jk"
new_substring = "sd"
string.reverse.sub(old_substring.reverse, new_substring.reverse).reverse
#=>  "jklazersdm"