A regular expression (also “regexp”, or just “reg”) consists of a
pattern and optional
flags.
There are two syntaxes that can be used to create a regular expression object.
The “long” syntax:
regexp = new RegExp("pattern", "flags");
And the “short” one, using slashes "/":
regexp = /pattern/; // no flags
regexp = /pattern/gmi; // with flags g,m and i (to be covered soon)
Slashes /.../ tell JavaScript that we are creating a regular expression. They play the same role as quotes for strings.
In both cases regexp becomes an instance of the built-in RegExp class.
The main difference between these two syntaxes is that pattern using slashes /.../ does not allow for expressions to be inserted (like string template literals with ${...}). They are fully static.
Slashes are used when we know the regular expression at the code writing time – and that’s the most common situation. While new RegExp is more often used when we need to create a regexp “on the fly” from a dynamically generated string. For instance:
