add regex notes

Author: nimonian

src/js/regex.md

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+# Regular expressions
+
+Regular expressions, or 'regex,' are patterns used to match character
+combinations in strings. In JavaScript, regex is a powerful tool for searching,
+replacing, and extracting text.
+
+## Basic syntax
+
+When we are searching some text for a particular pattern, we define the pattern
+in forward slashes like this:
+
+```js
+const pattern = /cat/
+```
+
+We can use `test()` to check whether a pattern exists in a string. It returns a
+boolean indicating whether the pattern is found.
+
+```js
+const text = 'The cat sat on the mat.'
+const pattern = /cat/
+pattern.test(text) // true
+```
+
+## Dot
+
+The `.` metacharacter matches any single character except a new line.
+
+The pattern below will match `'cat'`, `'mat'`, `'bat'` or anything else which
+looks like 'a single character followed by `'at'`'.
+
+```js
+const pattern = /.at/
+
+const text1 = 'cat'
+const text2 = 'cap'
+
+pattern.test(text1) // true
+pattern.test(text2) // false
+```
+
+## Searching for a pattern
+
+To do a **global** search for this pattern through some text, we add the `g`
+modifier and use the `match()` method:
+
+```js
+// add /g for global search
+const pattern = /.at/g
+
+const text = 'The cat sat on the mat.'
+text.match(pattern) // ['cat', 'mat']
+```
+
+## Quantifiers
+
+The `+` quantifier matches 1 or more occurences of the preceeding elements:
+
+```js
+const text = 'ac, abc, abbc, abbbc'
+const pattern = /ab+c/g
+text.match(pattern) // ['abc', 'abbc', 'abbbc']
+```
+
+Similarly,
+
+- the `*` quantifier matches 0 or more occurences
+
+- the `?` quantifier matches 0 or 1 occurences
+
+## Matching letters and digits
+
+We can use `\d` to match any digit character:
+
+```js
+const text = 'Order 66 was executed at 4:00 PM.'
+const pattern = /\d+/g
+text.match(pattern) // ['66', '4', '00']
+```
+
+We can use `\w` to match any word character (letters, digits, underscores):
+
+```js
+const tweet = 'Learning JavaScript is fun! #coding #JavaScript #100DaysOfCode'
+const pattern = /#\w+/g
+const hashtags = tweet.match(pattern)
+console.log(hashtags) // ['#coding', '#JavaScript', '#100DaysOfCode']
+```