Regular Expression

If you want to match strings that start with hey, use the ^ operator:
    /^hey/.test('hey') //✅ 
    /^hey/.test('bla hey') //❌
If you want to match strings that end with hey, use the $ operator:
    /hey$/.test('hey') //✅ 
    /hey$/.test('bla hey') //✅ 
    /hey$/.test('hey you') //❌
Combine those, and match strings that exactly match hey, and just that string:
    /^hey$/.test('hey') //✅

\d matches any digit, equivalent to [0-9]
\D matches any character that’s not a digit, equivalent to [^0-9]
\w matches any alphanumeric character, equivalent to [A-Za-z0-9]
\W matches any non-alphanumeric character, equivalent to [^A-Za-z0-9]
\s matches any whitespace character: spaces, tabs, newlines and Unicode spaces
\S matches any character that’s not a whitespace
\0 matches null
\n matches a newline character
\t matches a tab character
\uXXXX matches a unicode character with code XXXX (requires the u flag)
. matches any character that is not a newline char (e.g. \n) (unless you use the s flag, explained later on)
[^] matches any character, including newline characters. It’s useful on multiline strings.

? require zero or one item (optional)
    /^\d?$/
    
+ Match one or more (>=1) items
    /^\d+$/.test('144343') //✅ 
    /^\d+$/.test('') //❌ 
    
* Match 0 or more (>= 0) items
    /^\d*$/.test('') //✅ 
    /^\d*$/.test('1a') //❌

{n} Match exactly n items
    /^\d{3}$/.test('123') //✅ 
    /^\d{3}$/.test('12') //❌ 
    /^\d{3}$/.test('1234') //❌ 
    /^[A-Za-z0-9]{3}$/.test('Abc') //✅
    
{n,m} Match between n and m times:
     /^\d{3,5}$/.test('123') //✅ 
    /^\d{3,5}$/.test('1234') //✅ 
    /^\d{3,5}$/.test('12345') //✅ 
    /^\d{3,5}$/.test('123456') //❌

() create group
    /^(\d{3})(\w+)$/.test('123') //❌ 
    /^(\d{3})(\w+)$/.test('123s') //✅ 

()? optional group
    /^(\d{3})(\s)?(\w+)$/.exec('123 s') 
    //Array [ "123 s", "123", " ", "s" ] 
    /^(\d{3})(\s)?(\w+)$/.exec('123s') 
    //Array [ "123s", "123", undefined, "s" ]
    
Named capturing groups
    const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/ 
    const result = re.exec('2015-01-02') 
    // result.groups.year === '2015'; 
    //result.groups.month === '01'; 
    // result.groups.day === '02';
    
g: matches the pattern multiple times
i: makes the regex case insensitive
m: enables multiline mode. In this mode, ^ and $ match the start and end of the whole string. Without this, with multiline strings they match the beginning and end of each line.
u: enables support for unicode (introduced in ES6/ES2015)
s: (new in ES2018) short for single line, it causes the . to match new line characters as well.

\b matches a set of characters at the beginning or end of a word
\B matches a set of characters not at the beginning or end of a word

    'I saw a bear'.match(/\bbear/) //Array ["bear"] 
    'I saw a beard'.match(/\bbear/) //Array ["bear"] 
    'I saw a beard'.match(/\bbear\b/) //null 
    'cool_bear'.match(/\bbear\b/) //null