Regex Flashcards
What are the two ways to create regular expressions in JS?
Constructor: new RegExp(pattern[,flags])
e.g.
let regex = new RegExp('abc','i');
Literal: /pattern/flags
e.g.
let regex = /abc/i;
Both expressions would match against a sequence of a then b then c in a string ignoring case e.g. Abcre, baBC etc.
I want to know if the word ‘lite’ appears in a sentence and get a simple true/false response.
What regular expression method would I use?
test
e.g.
let regex = /lite/
let sentence = ‘ no such lite light’
let isPresent = regex.test(sentence)
isPresent = true;
How would I search the string ‘the quick brown Fox’ for the letter ‘f’ no matter the case?
let regex = /f/i
regex.test(‘the quick brown Fox’)
How would I get an array of all the capital letters in the sentence “The capital of Scotland is Edinburgh”.
let regex = /[A-Z]/g //all capital letters, do not stop at first match. let result = "The capital of Scotland is Edinburgh.".match(regex); result = ['T', 'S', 'E' ];
How would the result change if I removed the global flag from this regular expression operation?
let result = “17 Mall Road”.match(/\d/g);
The result of “17 Mall Road”.match(/\d/g) is [“1”, “7”].
Without the global flag it would only return the first match i.e. the number 1.
Write a regular expression that matches words containing bar and car.
/b|car/
or
/[bc]ar/
What is a character set?
A character set is a way to match different characters in a single position. It matches any string that contains any of the characters within the brackets
e.g. /[bcd]/
would match anyword containing b, c or d..
It is similar to or i.e. (b | c | d)
What are the different ways that the carat symbol ^ can be used in regular expressions?
- At the beginning of a character set, within the brackets e.g. /[^kl]e/
Here it means NOT k, NOT L. It matches anything NOT included in the character set.
- At the beginning of the regex proceeding the regex. e.g. /^p/
Here it means the string must start with p and will match ‘poll’ but not ‘nap’, for example.
Write a regular expression to find lowercase letters.
/[a-z]/g
This is a range. Instead of writing [abcdefg…etc.], you can use this shorthand.
Write a regular expression to find any digit character.
/[0-9]/
OR
/\d/
\d is a meta character, a shorthand for writing [0-9]
What will the following regex match?
/\s/
Any whitespace characters e.g. spaces, tabs
What is a word character?
Write a regular expression to find any word characters?
A word character is any alphanumeric character (letters and digits) in addition to the underscore i.e.
/[a-zA-Z0-9_]/
or
/\w/
\w is a meta character
Write a regular expression to match any non digit character.
/[^0-9]/
OR
/\D/
Write a regular expression to match any non word character.
/[^a-zA-Z0-9]/
OR
/\W/
What meta character matches any character except for a new line?
full stop i.e. .
Write a regular expression to match a tab character only.
/\t/
Write a regular expression to match any non space character.
/\S/
What does this mean?
const regex = /\d+/;
Match any string containing at least one but possibly more than one digit.
+ is a quantifier and matches the preceding expression one or more times.
What does this match?
const regex = /go*d/;
Matches ‘gd’ and ‘god’ and ‘gooood’ i.e. any word that contains g and d and any number of ‘o’s in the middle (possibly zero).
- is a quantifier and matches the preceding expression 0 or more times.
What does this mean? What does it match?
const regex = /goo?d/;
Matches ‘god’ and ‘good’ but not ‘goood’.
? is a quantifier and matches the preceding expression 0 or 1 time only.
How is the quantifier ‘$’ used in a regular expression?
Anything preceding $ should match the end of the string
e.g. const regex = /.com$/
This matches any string ending in ‘com’.
What does this match?
/go{2}d/
It matches only ‘good’
{N} is a quantifier and matches exactly N occurences of the preceding expression.
What does this match?
/go{2,}d/
It matches words containing a string starting with g and ending with d with at least two ‘o’s in between e.g. ‘good’, ‘gooood’ etc. but not ‘god’.
{N,} is a quantifier and matches AT LEAST N occurences of the preceding expression
What does this match?
/go{1,2}d/
It matches ‘god’ and ‘good’ only.
{N,M} is a quantifier and matches at least N and AT MOST M occurences of the preceding expression.
What’s the difference between:
/a+b/
and
/a+b/
/a+b/ will match any strings with one or more ‘a’s finishing with b e.g. ‘ab’, ‘aab’ but not ‘b’ and not ‘a+b’.
/a+b/ will match the string ‘a+b’
Here \ is used to escape the special character ‘+’
Write a regular expression to match any 10 numbers.
/^\d{10}$/
Use ^ and $ to enforce that the match should span the whole string i.e. the string cannot be more than 10 characters.
Use \d to identify digits.
Use the quantifier {10} to specify exactly 10 digits
Write a regular expression to match a data with the format DD-MM-YYYY or DD-MM-YY.
/^(\d{2}-){2}-\d{2}(\d{2})?$/
Write a regular expression to match with a format like abc.def.ghi.jkl (that exact number of dots) where each variable a, b, c, d, e, f, g, h, i, j, k, l can be any character except new line
/^(.{3}.){3}.{3}$/
Use ^ and $ to enforce that the match should span the whole string i.e. be made up of 15 characters.
Use . to identify any character except a new line and . to identify a dot/full stop.
Use the quantifier {3} to match an expression 3 times.
Write a function to return the number of vowels in a string using regex.
const vowels = str => str.match(/[aeiou]/ig)?.length ?? 0
Match returns null if no match found or an array of all the matches.
Write a function to capitalize the first letter of every word in a sentence.
const capitalize = sentence => sentence.split(/\s/g).map(word => word.substring(0,1).toUpperCase() + word.substring(1)).join(“ “)
