Please explain RegExp javascript behavior

Question

I'm experiencing some understanding problem of javascript RegExp behavior. Please help me to get how this stuff works.

var str = "_prefix_123 blah blah _prefix_321 blah blah blah _prefix_100500";
var prefix = "_prefix_";
var exp = new RegExp (prefix+"\d*","gm");

str1 = str.replace(exp,"hello");
alert(str1);

str2 = str.replace(/_prefix_\d*/gm, "hello");
alert(str2);

check the fiddle here http://jsfiddle.net/6PSdg/4/

For some reason two identical (at least they seem like) regex patterns return different results. Can anyone explain what's going on? ))

Thanx!

T.J. Crowder · Accepted Answer · 2013-03-15 11:55:00Z

2

The difference is that when you use the RegExp constructor, you're passing in a string, and backslashes in string literals are interpreted as string escapes. So to actually pass a backslash to the regex engine, you have to make sure the backslash in the string is escaped (by putting a backslash in front of it):

var exp = new RegExp (prefix+"\\d*","gm");
// Here ----------------------^

Updated Fiddle

answered Mar 15, 2013 at 11:55

T.J. Crowder

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