Regular expression (regex)

regex is a sequence of characters that specifies a search pattern in text

Syntax

Identifiers:

• .: any character, except for a newline

• \d: any digit (0-9)

• \D: not a digit

• \w: any character (a-z, A-Z, 0-9, _)

• \W: not a character

• \s: whitespace (space, tab, newline)

• \S: not whitespace

• \b: the whitespace around words (word boundary)

• \B: not a word boundary

Modifiers:

• +: match 1 or more

• ?: match 0 or 1

• *: match 0 or more

• ^: match the beginning of a string

• $: match the end of the string

• |: either or

• [ ]: range or variance [A-Z] (match characters in brackets)

• [^ ]: not match the range or variance [A-Z]

• {x}: expecting x amount

• {1,3}: we're expecting 1 to 3

White Space Characters:

• \n: new line
• \s: space
• \t: tab
• \e: escape
• \f: form feed
• \r: return

DON't FORGET!: { } \

syntax characters:

• \:
• .:
• *:
• +:
• ?:
• (:
• ):
• [:
• ]:
• {:
• }:

Groups & Look-around

• (abc): capture group
• \1: back-reference to group #1
• (?:abc): non-capturing group
• (?=abc): positive lookahead
• (?!abc): negative lookahead

Quantifiers

• ?: match never or 1

• *: match 0 or more times

• +: match 1 or more times

• {n}: match n times

• {n,}: match n or more times

• {n,m}: match at least n times, at most m times

• |: match this or that

Atoms

Atoms are the basic building blocks of regular expressions

Lookahead

JavaScript

const regTerm = /hello/;

console.log(regTerm);
console.log(regTerm.source); // hello

Flags

Literal flag	Property name	ES	Description
d	hasIndices	ES2022	Switch on match indices
g	global	ES3	Match multiple times
i	ignoreCase	ES3	Match case-insensitively
m	multiline	ES3	^ and $ match per line
s	dotAll	ES2018	Dot matches line terminators
u	unicode	ES6	Unicode mode (recommended)
y	sticky	ES6	No characters between matches

Methods

• exec(): executes a search for a match in a specified string and returns a result array, or null

  const regex1 = RegExp("foo*", "g");
  const str1 = "table football, foosball";
  let array1;
  
  while ((array1 = regex1.exec(str1)) !== null) {
    console.log(`Found ${array1[0]}. Next starts at ${regex1.lastIndex}.`);
    // expected output: "Found foo. Next starts at 9."
    // expected output: "Found foo. Next starts at 19."
  }

• test(): executes a search for a match between a regular expression and a specified string. Returns true or false

  const str = "table football";
  
  const regex = new RegExp("foo*");
  const globalRegex = new RegExp("foo*", "g");
  
  console.log(regex.test(str));
  // expected output: true