The JavaScript version.

• Metacharacters
  • .: Any one character except newline, same as [^\n].
  • \d, \D: Any one digit/non-digit character (where digits are [0-9]).
  • \w, \W: Any one word/non-word character. For ASCII, word characters are [a-zA-Z0-9_].
  • \s, \S: Any one space/non-space character. For ASCII, whitespace characters are [ \n\r\t\f].
• Occurrence Indicators
• +: One or more, e.g. [0-9]+ matches 1 or more digits, such as "123", "0000".
  • *: Zero or more (accepts the above + empty strings).
  • ?: Zero or one (optional), e.g., [+-]? matches an optional "+", "-", or an empty string.
  • {}
    • {m,n}: m to n (both inclusive).
    • {m}: Exactly m times.
    • {m,}: m or more times (m+).
• Position Anchors
  • ^: Start of line, e.g. ^[0-9]$ matches a numeric string.
  • $: End of line
  • \b: Boundary of word, i.e., start-of-word or end-of-word. E.g., \bcat\b matches the word "cat" in the input string.
  • \B: Inverse of \b, i.e. non-start-of-word or non-end-of-word.
• Parenthesized Back References
  • ():
• Character Class (or Bracket List)
  • []
  • [...]: Accept any one of the character within the bracket.
  • [.-.]: Accept any one of the characters in the range, e.g. [0-9], [A-Za-z].
  • [^...]: Rejects any one of the character, e.g. [^0-9] matches any non-digit.
  • Only ^, -, ], \ require escape sequence inside the bracket list.
• |: OR operator, e.g. four|4 accepts "four" or "4".
• \: Escape sequence to accept a char with special meaning in regex.
  • Regex recognizes common escape sequences such as \n for newline, \t for tab, \r for carriage-return, \nnn for a up to 3-digit octal number, \xhh for a two-digit hex code, \uhhhh for a 4-digit Unicode, \uhhhhhhhh for a 8-digit Unicode.

• https://www3.ntu.edu.sg/home/ehchua/programming/howto/Regexe.html#:~:text=On%20the%20other%20hand%2C%20the,%5CD%20or%20non%2Ddigit.