feat: add regular expressions cheat sheet

javascript/regular-expressions.md

@@ -0,0 +1,68 @@
+# Regular Expressions
+
+## Basic Example
+
+```javascript
+const regex = /[A-Z]/g
+
+const anotherRegex = new RegExp(/[A-Z]/, 'g')
+
+const yetAnotherRegex = new RegExp('[A-Z]', 'g')
+
+anotherRegex.test('Apple') // true
+anotherRegex.test('apple') // false
+
+```
+
+## Cheat Sheet
+
+Source: [RegExr](https://regexr.com/)
+
+### Character classes
+
+| Symbol | Meaning |
+| --- | --- |
+| `.` |	any character except newline |
+| `\w\d\s` |	word, digit, whitespace |
+| `\W\D\S`	| not word, digit, whitespace |
+| `[abc]`	| any of `a`, `b`, or `c` |
+| `[^abc]` | not `a`, `b`, or `c` |
+| `[a-g]` | character between `a` & `g` |
+
+### Anchors
+
+| Symbol | Meaning |
+| --- | --- |
+| `^abc$`	| start / end of the string |
+| `\b\B`	| word, not-word boundary |
+
+### Escaped characters
+
+| Symbol | Meaning |
+| --- | --- |
+| `\.\*\\`	| escaped special characters |
+| `\t\n\r`	| tab, linefeed, carriage return |
+
+### Groups & Lookaround
+
+| Symbol | Meaning |
+| --- | --- |
+| `(abc)`	| capture group |
+| `\1`	| backreference to group #1 |
+| `(?:abc)`	| non-capturing group |
+| `(?=abc)` |	positive lookahead |
+| `(?!abc)`	| negative lookahead |
+
+### Quantifiers & Alternation
+
+| Symbol | Meaning |
+| --- | --- |
+| `a*a+a?` |	0 or more, 1 or more, 0 or 1 |
+| `a{5}a{2,}`	| exactly five, two or more |
+| `a{1,3}`	| between one & three |
+| `a+?a{2,}?`	| match as few as possible |
+| `ab\|cd`	| match ab or cd |
+
+## Resources
+
+* [RegExr](https://regexr.com/): An online tool that lets you test a regex against different strings. You can see where the matches are, and it breaks down what each part of the regex is looking for.