Replace special characters in a string with underscore (_) in JavaScript

Last Updated : 23 May, 2019

JavaScript replace() method is used to replace all special characters from a string with _ (underscore) which is described below:

replace() method: This method searches a string for a defined value, or a regular expression, and returns a new string with the replaced defined value.
Syntax:
```
string.replace(searchVal, newvalue)
```
Parameters:
- searchVal: It is required parameter. It specifies the value, or regular expression, that is going to replace by the new value.
- newvalue: It is required parameter. It specifies the value to replace the search value with.
Return value: It returns a new string where the defines value has been replaced by the new value.

Example 1: This example replaces all special characters with _ (underscore) using replace() method.

<!DOCTYPE HTML> 
<html> 
    <head> 
        <title> 
            Replace special characters in a
            string with _ (underscore)
        </title>
    </head> 
      
    <body style = "text-align:center;"> 
      
        <h1 style = "color:green;" > 
            GeeksForGeeks 
        </h1> 
          
        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;">
        </p>
          
        <button onclick = "gfg_Run()"> 
            Click here
        </button>
          
        <p id = "GFG_DOWN" style = 
            "color:green; font-size: 20px; font-weight: bold;">
        </p>
          
        <script>
            var el_up = document.getElementById("GFG_UP");
            var el_down = document.getElementById("GFG_DOWN");
            var str = "This, is# GeeksForGeeks!";
            el_up.innerHTML = str;
              
            function gfg_Run() {
                el_down.innerHTML = 
                    str.replace(/[&\/\\#, +()$~%.'":*?<>{}]/g, '_');
            }         
        </script> 
    </body> 
</html>                    

Output:

Before clicking on the button:
After clicking on the button:

Example 2: This example replace a unique special character with _ (underscore). This example goes to each character and checks if it is a special character that we are looking for, then it will replace the character. In this example the unique character is $(dollar sign).

<!DOCTYPE HTML> 
<html> 
    <head> 
        <title> 
            Replace special characters with
            _ (underscore)
        </title>
    </head> 
      
    <body style = "text-align:center;"> 
      
        <h1 style = "color:green;" > 
            GeeksForGeeks 
        </h1> 
      
        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;">
        </p>
          
        <button onclick = "gfg_Run()"> 
            Click here
        </button>
          
        <p id = "GFG_DOWN" style = 
            "color:green; font-size: 20px; font-weight: bold;">
        </p>
          
        <script>
            var el_up = document.getElementById("GFG_UP");
            var el_down = document.getElementById("GFG_DOWN");
            var str = "A$computer$science$portal$for$Geeks";
            el_up.innerHTML = str;
              
            function gfg_Run() {
                  
                var newStr = "";
                  
                for(var i=0; i<str.length; i++) { 
                    if (str[i] == '$') {
                        newStr += '_';
                    }
                    else {
                        newStr += str[i];
                    }
                }
                el_down.innerHTML = newStr;
            }         
        </script> 
    </body> 
</html>                    

Output:

Before clicking on the button:
After clicking on the button:

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