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I am using Nodejs to build application in which I need to process certain strings I have used the JS "RegExp" object for this purpose. I want only a part of my string in the regex to be case insensitive

var key = '(?i)c(?-i)ustomParam';
var find = '\{(\\b' + key +'\\b:?.*?)\}';
var regex = new RegExp(find,"g");

But it breaks with following error

SyntaxError: Invalid regular expression: /{(\b(?i)c(?-i)ustomParam\b:?.*?)}/

I will get the key from some external source like redis and the string to be matched from some other external source , I want that the first alphabet should be case-Insensitive and the rest of alphabets to be case-Sensitive.

When I get the key from external source I will append the (?i) before the first alphabet and (?-i) after the first alphabet.

I even tried this just for starters sake, but that also didn't work

var key ='customParam';
var find = '(?i)\{(\\b' + key +'\\b:?.*?)\}(?-i)';
var regex = new RegExp(find,"g");

I know I can use "i" flags instead of above ,but that's not my use case. I did it just to check.

TylerH
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Vipresh
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    You cannot make a _part_ of regex case-insensitve, use `i` flag on complete regex. – Tushar May 13 '16 at 10:45
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    Bad news: even [XRegExp cannot offer this functionality](http://xregexp.com/syntax/), you can only use `(?i)` on the whole pattern declaring it at the pattern beginning. In XRegExp, you can define the regex as `var regex = XRegExp('(?i)\\{(\\b' + key +'\\b:?.*?)\\}', 'g');` – Wiktor Stribiżew May 13 '16 at 10:50
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    OKi so JavaScript regex library sucks. @WiktorStribiżew if you had posted your comment as answer I would have accepted it as answer – Vipresh May 13 '16 at 11:03

1 Answers1

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JavaScript built-in RegExp does not support inline modifiers, like (?im), let alone inline modifier groups that can be placed anywhere inside the pattern (like (?i:....)).

Even XRegExp cannot offer this functionality, you can only use (?i) on the whole pattern declaring it at the pattern beginning.

In XRegExp, you can define the regex ONLY as 

var regex = XRegExp('(?i)\\{(\\b' + key +'\\b:?.*?)\\}', 'g');

On May 27, 2020, still neither JavaScript native RegExp, nor XRegExp patterns support inline modifier groups (i.e. (?i:...)), nor placing them in any part of the pattern (as far as XRegExp is concerned).

Wiktor Stribiżew
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