IsNumeric() method works for only Integer value not for decimal value

like this

String str = '10';
system.debug('===strToDec=='+str.isNumeric());

This will return true.

String str = '10.25';

Decimal strToDec = decimal.valueOf(str);

system.debug('===strToDec=='+strToDec);

String decToStr = String.valueOf(strToDec);

system.debug('===decToStr=='+decToStr);

Run this code in developer console ad check output.

In you case do something like. First convert string to numeric then check and again convert string into decimal for future use

String  str = '10.25';
Integer intCheck = Integer.ValueOf(str);
Decimal decVal = Decimal.ValueOf(str);
if(String.ValueOf(intCheck ).isNumeric())
//use decVal here

Don't understand why you need this.

The format method automatically does the commas and periods or visa versa (depending on whether your profile is in the US or elsewhere)

Assuming the $ sign applies to all cases in your org you can simply use

Decimal input = 2000;
String output = '$' + String.valueOf(input.format());

Note the String.valueOf(). This is required because the input variable is defined as a Decimal.

Edit: I noticed a bug with the Decimal.format() method where it will not show any numbers past the decimal point if there are only zeros there, ex. $100.00. To solve this I came up with this method.

private String getCents(Decimal x){
    String y = String.valueOf(x);
    String z = '.';
    if(y.contains(',')) z = ',';
    y = y.substring(0, y.indexOf(z));
    if(x - Decimal.valueOf(y) == 0)
        return String.valueOf(x.format()) + z + '00';
    else return String.valueOf(x.format());
}

Then to update the example above, it would be:

Decimal input = 2000;
String output = '$' + getCents(input);

Consider using a regular expression and the replaceAll() method.

The following expression will identify any character that is not a decimal point or a digit. You can try it out at regex101.com

[^.\d]

In Apex you'll need to escape the slash...

[^.\\d]

By this you'll be able to strip out the comma without having to go searching for it.

String value = '12,345.678';
system.assertEquals('12345.678', value.replaceAll('[^.\\d]','');
system.assertEquals(12345.678, Decimal.valueOf(value.replaceAll('[^.\\d]',''));

This cleaned-up value should convert to a Decimal without giving an error. You may also find it helpful to create a utility method that takes the String to be converted as well as a Decimal value to return if an error occurs.

The + is used to add two numbers together, and to concatenate String values. When you use a non-string next to a String, toString() will be called on the object. As such, you can simply say:

System.debug(a.get('Name')+' '+a.get('AnnualRevenue'));

You can't directly cast a non-String to a string, as the following is also an error:

String output = (String)1.23;

But you can use the + operator with a string on either side:

String output = ''+1.23;

String output = 1.23+'';

I often use this as a quick technique to convert most primitives to a String.

You can only use typecasting between compatible types, like a Decimal to an Integer (with precision loss) or a String to an Id.