Regular Expression Cheat Sheet

regularExpression.md

I'm documenting the .match() method for JavaScript, there are others.

• / by itself indicates the beginning and the end of the regex
• /^ combined with $/ means “ match the entire string” (rather than matching substrings that begin and/or end somewhere in the middle)
• the i suffix is a modifier indicating that the match should be done in a case-insensitive manner
• [a-z0-9\-] matches a character which is either a number, letter or dash (works for capital letters because of the i at the end
• the + indicates the match one or more of whatever came before
• if you put a caret (^) inside of the match expression capturer thingamajig, it reverses the meaning

// Match any 3 character string where the first character is a number from 1-9 and the second two characters are numbers from 0-9.
var IS_3_DIGIT_FINITE_INTEGER = /^[1-9][0-9][0-9]$/

// Match any 3 character string where the first character is ANYTHING OTHER THAN a number from 1-9, and the second two characters are ANYTHING OTHER THAN numbers from 0-9.
var OPPOSITE_OF_THAT_SORT_OF = /^[^1-9][^0-9][^0-9]$/

if (!_.isString(req.param('username')) || req.param('username').match(/[^a-z0-9]/i) || req.param('username').length === 0) {
  return res.badRequest('Invalid username');
}
// …

that says:

• if it is not a string
• OR it contains ANY characters other than letters or digits
• OR it is the empty string then it is invalid.

The original if statement said:

• if it is not a string
• OR it IS NOT a string CONSISTING ENTIRELY of one or more characters which are all either letters or digits then it is invalid.

• remember ^ means two different things
• /^ means the start of the string
• [^abc] means a single character which is not a, b, or c
• + means one or more of the previous character or match group thing
• * means zero or more of the previous character or bracket match group thing
• . means any character
• Use \ to escape those things (as well as brackets, parentheses, and slashes)