JavaScript

Regular Expressions

RegExp consist of a pattern an optional flags. RegExp can be expressed using short or long syntaxes above.

Standard behaviour

• Only search for 1 match.
• Case-sensitive

Flags

There are only 6 flags in JavaScript:

• i: case-insensitive
• g: global. Looking for all matches.
• m: multiline mode
• s: "dotall" mode. Allowing a dot . to match newline character\n.
• u: full Unicode support
• y: Searching at exact opsition in the text.

RegExp cheatsheets:

• MDN Web Docs: Regular expression syntax cheatsheet
• Debuggex: JavaScript Regex Cheatsheet

Please also check: Capturing group.

Dinamically generated RegExp

The long new RegExp() syntax allows expression to be inserted, for example with ${...}, and therefore allow dinamically generated RegExp. This is not possible for the short syntax using slashes //. It can only contain static expression, that's known already duting the time the code is written. Which is most of the cases.

let tag = prompt("User tag:", "Kitten");
let tagRegex = new RegExp(`<${tag}>`);
tag;            "Kitten"
tagRegex;       /<Kitten>/

Searching using match()

Return values:

• With flag g:
  Return an array of all matches.
• Without flag g:
  Return an array of the first match, with the full match at index 0 and some additional details in properties.
  The array might have index other than 0.
• No match (with or without flag g):
  Return null (instead of an empty array).

Replacing using replace()

Default method only replace the first match. To replace all matches, use flag g. Case-sensitive. Use flag i to make case-insensitive.

The replacement is in a string format "". We can insert fragment of the match using special character combinations below:

• $&: insert the whole match.
• $`: insert a part of the string before the match.
• $': insert a part of the string after the match.
• $n: with n a 1-2 digit number. Inserts the contents of n-th parentheses.
• $<name>: insert the content of the parentheses with the given name.
• $$: insert character $.

text = "cat dog lion";
text.replace(/dog/, "pig");     // 'cat pig lion'
text.replace(/dog/, "$& pig");  // 'cat dog pig lion'
text.replace(/dog/, "$` pig");  // 'cat cat  pig lion'
text.replace(/dog/, "$' pig");  // 'cat  lion pig lion'

testing using test()

test()> looks for at least 1 match. If found, returns true. Otherwise, false.

statement = "I like dogs. Cats though, I love best.";
regex1 = /cat/i;
regex1.test(statement);     // true
regex2 = /lion/i;
regex2.test(statement);     // false