passing variable to a regexp in javascript

Question

I have a msg like

Max {0} chars allowed in {1}

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 its not able to take the param "i" to create the reg exp.

Whats the way to achieve the same ?

Answer 1

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.

Answer 2

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 3

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