Regular Expressions in JavaScript: A crash course

I frequently find myself trying to quickly explain the basics of JavaScript regexp, and doing a hasty or incomplete job. Instead, I'm making this. It is by no means a complete reference. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp for that.

Common syntax

Things that match one character

• . - any character
• [abc] - any character that matches one of the characters inside the brackets (in this case "a", "b", or "c")
• [^abc] - any character that does not match any of the other characters inside the brackets (in this case "a", "b", or "c")
• \s - any whitespace character
• \S - any non-whitespace character

Symbols that modify whatever the thing before them matches

• + - one or more occurrences of whatever came before
• * - zero or more occurrences of whatever came before
• ? - zero or exactly one occurrences of whatever came before

Symbols that indicate whether this is a "entirely matches", "starts with", "ends with", or "contains" kind of a check

• ^ - at the beginning of your regexp means "from the start"
• $ - at the end of your regexp means "through the end"

Putting it together

• [a-z0-9] - an alphanumeric character (lowercase) e.g. m or 4
• [^a-z0-9] - the opposite-- any character which is not alphanumeric (lowercase) e.g. D or &
• [a-z0-9]+ - one or more alphanumeric characters (lowercase) e.g. m393mkeeu13 or 494913581 or asdgasgd
• washroo+m? - the word "washroom" but allowing for extra "o"s and with the "m" being optional. e.g. washroom or washroooooooooom or washroooo.

Simple examples

• /dogfood/ => matches any string that contains "dogfood" -- e.g. "I like to eat dogfood." WILL MATCH.
• /^dogfood$/ => matches "dogfood". The entire string must be "dogfood". Nothing else matches.
• /^dogfood/ => matches any string that starts with "dogfood" -- e.g. "dogfood is ok."
• /dogfood$/ => matches any string that ends with "dogfood" -- e.g. "I like to eat dogfood" (notice no period)

A handful of slightly more complex examples

• /^[^A-Z0-9]/ => matches any string that STARTS with any character OTHER THAN an uppercase alphabet character or a numeral. e.g. "¿I like to eat dogfood" (because of the ¿)
• /^[A-Z0-9]/ => matches any string that STARTS with any uppercase alphabet character or a numeral e.g. "I like to eat dogfood" (because of the I)