Regex, short for regular expression, is a sequence of characters that defines a search pattern. It's a powerful tool used in computer programming and text processing to find, match, and manipulate strings of text. With regex, you can search for specific patterns within a larger string, such as finding all email addresses in a document or validating the format of a phone number. Regex is supported by many programming languages, including JavaScript, Python, and Java.

Here's an example of using regex in JavaScript to validate a simple email address:

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const emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;

const email = "test@example.com";
if (emailPattern.test(email)) {
  console.log("Valid email address!");
} else {
  console.log("Invalid email address!");
}

In this example, the emailPattern regex defines the pattern for a valid email address. The test() function is used to check if the email string matches the pattern. If it does, the program will print "Valid email address!". Otherwise, it will print "Invalid email address!".

Regular expressions, also known as regex or regexp, are patterns used to match character combinations in strings. In JavaScript, regular expressions are created using the RegExp object or by using regex literals (appearing between forward slashes). Here are some examples:

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// RegExp object example
const regex = new RegExp('hello');
const string = 'hello world';
console.log(regex.test(string)); // true

// Regex literal example
const regex2 = /world/;
const string2 = 'hello world';
console.log(regex2.test(string2)); // true

In these examples, we create regular expressions to match the strings "hello" and "world". We then use the .test() method to check if a given string contains a match for the regular expression.

Regular expressions can contain special characters and character classes to define more complex patterns. For example:

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const regex3 = /\d+/; // matches one or more digits
const string3 = 'the code is 1234';
console.log(regex3.test(string3)); // true

const regex4 = /\w{3}/; // matches three word characters
const string4 = 'the cat in the hat';
console.log(regex4.test(string4)); // true

In these examples, we use the special characters \d to match digits and \w to match word characters (letters, digits, and underscores). We also use quantifiers like + and {3} to match one or more of the preceding token.

Regular expressions can also use groups and capturing to extract specific parts of a matched string. For example:

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const regex5 = /(\d{3})-(\d{2})-(\d{4})/; // matches a date in format XXX-XX-XXXX
const string5 = 'my birthdate is 123-45-6789';
const match = string5.match(regex5);
console.log(match); // [ '123-45-6789', '123', '45', '6789', index: 18, input: 'my birthdate is 123-45-6789', groups: undefined ]
console.log(match[1]); // '123'
console.log(match[2]); // '45'
console.log(match[3]); // '6789'

In this example, we use parentheses to create groups that match specific parts of the date (the three groups match the month, day, and year). We then use the .match() method to get an array of matches and use array indexing to extract the specific groups.

Regular expressions can be powerful tools for string manipulation and data extraction, but they can also be complex and difficult to read. It's important to carefully consider the use case and user input to avoid common regex pitfalls like catastrophic backtracking. For more information, see the MDN docs on regular expressions: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions.

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# import necessary libraries
import requests
from bs4 import BeautifulSoup
import re
import gspread
from oauth2client.service_account import ServiceAccountCredentials

# ask user for inputs
web_address = input("Enter web address: ")
business_type = input("Enter business type: ")

# set up google sheet
scope = ['https://www.googleapis.com/auth/spreadsheets', 'https://www.googleapis.com/auth/drive.file', 'https://www.googleapis.com/auth/drive']
creds = ServiceAccountCredentials.from_json_keyfile_name('<PATH-TO-JSON-FILE>', scope) # replace <PATH-TO-JSON-FILE> with actual path
client = gspread.authorize(creds)
sheet = client.open('<SPREADSHEET-NAME>').sheet1 # replace <SPREADSHEET-NAME> with actual spreadsheet name
row = 1 # initialize row number for spreadsheet

# get page content
page = requests.get(web_address)
soup = BeautifulSoup(page.content, 'html.parser')

# extract business names and email addresses from page content
results = soup.find_all('a', href=True, text=re.compile(business_type))
for result in results:
    business_name = result.text
    email = None
    contact_page = requests.get(result['href'])
    contact_soup = BeautifulSoup(contact_page.content, 'html.parser')
    email_tags = contact_soup.find_all('a', href=True, text=re.compile('[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}'))
    if email_tags:
        email = email_tags[0]['href'][7:]
    # add business name and email to spreadsheet
    sheet.update_cell(row, 1, business_name)
    sheet.update_cell(row, 2, email)
    row += 1

Note: Replace <PATH-TO-JSON-FILE> with the actual path of your Google API credentials file and <SPREADSHEET-NAME> with the actual name of your Google Spreadsheet.

To take only numbers from a string and turn them into an integer in Python, you can use regular expressions and the int() function. Here's an example:

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import re

my_string = "I am 27 years old"

numbers = re.findall('\d+', my_string)  # finds all the numbers in the string
result = list(map(int, numbers))  # converts the numbers to integers and puts them in a list

print(result)  # prints [27]

In the code above, we import the re module to use regular expressions. Then, we define a string with some text and numbers. We use the re.findall() function to find all the numbers in that string and store them in the numbers variable. Finally, we use a list comprehension and the int() function to convert each number in the list to an integer.

Here's an example in JavaScript on how to replace spaces with a dash in a string:

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let myString = "Hello World";
let stringWithoutSpaces = myString.replace(/\s+/g, '-');

console.log(stringWithoutSpaces); // "Hello-World"

In the example above, we first declare a variable myString and set its value to "Hello World".

We then use the replace method on the myString variable, passing in a regex pattern as the first argument and the replacement string "-" as the second argument.

The regex pattern /\s+/g matches one or more whitespace characters globally in the string. By replacing each occurrence of one or more spaces with a dash, we get the desired output.

Finally, we log the stringWithoutSpaces variable to the console, which displays the string with spaces replaced by a dash: "Hello-World".

HTML

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<form>
  Email: <input type="text" name="email" id="email" />
  <input type="button" value="submit" onclick="validate()" />
</form>

JavaScript

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function validate(){
  const email = document.getElementById('email').value
  const regex = /^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/;
  if(regex.test(email)) {
    alert("Valid email!")
  } else {
    alert("Invalid email!")
  }
}

CSS

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input[type=text] {
  padding: 5px;
  border: 1px solid #ccc;
  box-shadow: 2px 2px 5px #ccc;
}

JavaScript has a built-in method, .toLowerCase(), which will convert any string to all lowercase letters. You can use this method to ensure that all strings are in the same format whenever they're compared.

For example, let's say you'd like to compare two strings as shown below:

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const str1 = "HAPPY";
const str2 = "happy";

The .toLowerCase() method can be used to convert the strings to the same format before comparing them:

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const isMatch = str1.toLowerCase() === str2.toLowerCase();
console.log(isMatch); // true

By converting the strings to lowercase, you can ignore capital letters and spelling, ensuring that potential mistakes don't ruin comparisons.

In addition, you can use .replace() to remove unnecessary spacing before comparing strings. For instance, if you receive strings with extra space:

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const str3 = " HAPPY ";
const str4 = "happy";

You could convert both strings to the same format using .replace() to remove the unnecessary spaces:

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const isMatch = str3.replace(/ /g, "").toLowerCase() === str4.replace(/ /g, "").toLowerCase();
console.log(isMatch); // true

Here, we use the .replace() method with a regex to remove all spaces (/ /g) and then follow that up with .toLowerCase() to make the strings all lowercase.

Using JavaScript, you can create a prompt question in which the user's input must both contain a capital letter and a lowercase letter. This can be achieved by using a Regular Expression within an if statement to ensure that the user's input matches the specific RegEx pattern you're looking for.

The following code example shows how this could be done:

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let answer = prompt("Please enter username");
const regex = /^(?=.*[a-z])(?=.*[A-Z])[a-zA-Z]+$/;

if (regex.test(answer)) {
  alert("Valid input! Thank you.");
} else {
  alert("Input must contain both capital and lowercase letters!");
}

For more information on Regular Expressions, please check out this guide.