JavaScript Regex return the last occurrence of y between x while ignoring double x's second occurrence

Question

Okay so the title must be a head scratcher, but here is the full string:

y = "_target"
x = "_"

string A = "prefix_param_name_id_set_selected_param_name_index__target_"
string B = "prefix_param_name_id_set_selected_param_name_index_target_"

regex looks for "y"
"y" will always be the value between the last occurrence of "x"
if the last occurrence of "x" is preceded with double "x" then regex assumes the second part of double "x" is part of "y"

so returning "y" where at one instance "y" may be "_target" and at another "y" may be "target"

This is where I am at:

var str = "prefix_param_name_id_set_selected_param_name_index__target_";
alert(str.match(/\(([^)]*)\)[^(]*$/)[1]);

returns "target"; it should be "_target".

UPDATE:

Please note that "y" is a variable and is unknown. str is known, but the regex does not know what "y" is, only that it is found between at the last occurrence of "x"

When I run `var str = "prefix_param_name_id_set_selected_param_name_index__target_"; alert(str.match(/$([^)]*)$[^(]*$/)[1]);` I get null — chinloyal, Mar 14 '18 at 12:42

gurvinder372 · Answer 1 · 2018-03-14T13:42:29.550

0

You need to use x and y to create the regular expression

var y = "_target";
var x = "_";
var regexp = new RegExp("(?<=" + x + ")(" + y + ")(?=" + x + ")", "g");
var input = "prefix_param_name_id_set_selected_param_name_index__target_";
var matches = input.match(regexp);

matches outputs

["_target"]

Explanation

Use (?<=" + x + ") to check if y follows x
(" + y + ") captures y
(?=_) checks if x follows y as well.

Demo

var y = "_target";
var x = "_";
var regexp = new RegExp("(?<=" + x + ")(" + y + ")(?=" + x + ")", "g");
var input = "prefix_param_name_id_set_selected_param_name_index__target_";
var matches = input.match(regexp);
console.log(matches);

edited Mar 14 '18 at 13:42

answered Mar 14 '18 at 12:45

gurvinder372

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@zer00ne positive look-behind – gurvinder372 Mar 14 '18 at 12:50
I see, I am looking at it with JS eyes not as pcre – zer00ne Mar 14 '18 at 12:54
@zer00ne Sorry, *pcre*? – gurvinder372 Mar 14 '18 at 12:54
Perl Compatible Regular Expression is capable of look behind JS regex isn't. – zer00ne Mar 14 '18 at 12:58
Just keep in mind that "_target" is not known, it could be "_color" in one instance, and "_somethingelse" in another. – Jacques Joubert Mar 14 '18 at 13:14
You will have to create regex accordingly – gurvinder372 Mar 14 '18 at 13:18
where do you actually apply var regexp ? its defined, but never used. Or am I missing something? – Jacques Joubert Mar 14 '18 at 13:36
@JacquesJoubert Please check again, I have updated the code. – gurvinder372 Mar 14 '18 at 13:42
@gurvinder372 thank you for your input, appreciate it. Problem not solved yet, instead I decided a completely different approach. – Jacques Joubert Mar 28 '18 at 09:04

JavaScript Regex return the last occurrence of y between x while ignoring double x's second occurrence

1 Answers1