passing variable to a regexp in javascript [duplicate]

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How do you use a variable in a regular expression? (29 answers)

Is there a RegExp.escape function in JavaScript? (21 answers)

Closed 7 years ago.

Possible Duplicate:
Escape string for use in Javascript regex

I have a msg like this:

Max {0} chars allowed in {1}

And I have a function to create a message using the arguments passed as

for(var i = 0; i < agrs.length; i++){
 reg = new RegExp('\{'+i+'\}', 'gi');
 key = key.replace(reg,agrs[i])
}

The problem is that it's not able to take the param i to create the reg exp.

What's the way to achieve this?

• MandM: @casperOne, what is the duplicate? Neither this question nor this question were the same as this question. This question is important because it shows actually mixing a string variable into a regex pattern, whereas the other questions' answers only provided examples where escaping characters was not an issue.
• Daniel Reina: I agree with MandM. And given that this question has been marked by duplicate we cannot update the answers with ES6 JS, and people will keep using verbose JS. @casperOne can you unmark this as duplicate?

Answer 1

Your regexp is /{0}/gi since you create it from a string. And it is not a valid expression. You need to escape { in the regexp because it has a special meaning in the regexp syntax, so it should be:

new RegExp('\\{'+i+'\\}', 'gi');

which is /\\{0\\}/gi. You need to escape the escaping \\ in the string.

Answer 2

I would strongly encourage you to use the functional form of String.replace() to solve your problem, rather than trying to parametrize the regexp in a for-loop that iterates over {0},{1},etc.

In other words, rather than look for '{0}' or '{1}', just look for '{([0-9]+)}' (i.e. {}'s surrounding an arbitrary number, and pass a function to the replace() function to intelligently replace these expressions based on the number between the {}'s. This lets you use a RegExp literal which you can write as /{([0-9]+)}/ rather than mess around with escape characters in strings.

Something like this:

s='the song says {0} little {1} little {2} little-endians';
args=['zero','one','two'];
new_string = s.replace(/{([0-9]+)}/g, function(wholematch,firstmatch)
 {return args[(+firstmatch)]; }
);

which yields

the song says zero little one little two little-endians

See this similar question.

edit: if you want to leave alone items that are greater than the length of your args list, make sure to sanity-check the parameter number:

s='the song says {0} little {1} little {2} little-endians,\n'+
 ' {3} little {4} little {5} little-endians';
args=['zero','one','two'];
new_string = s.replace(/{([0-9]+)}/g, function(wholematch,firstmatch)
 {var i = +firstmatch; return i < args.length ? args[i] : wholematch;}
);

which yields

the song says zero little one little two little-endians,
 {3} little {4} little {5} little-endians

Answer 3

I think your second line is supposed to be

reg = new RegExp('\{'+i+'\}', 'gi');

but I don't know if that's your only problem or not.

• Nrj: Yep it is like that, but it does not allow to create one. Says "invalid quantifier".

Answer 4

function escapeRegExp(str) {
 return str.replace(/[-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
}
var re = new RegExp(escapeRegExp(str)); 

See: Escape string for use in Javascript regex