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regular-expressions.md

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➲ Regular Expressions:

Regular expressions also known as Regex. These are a powerful tool used for pattern matching and text manipulation. It provide a flexible way to find, replace, and validate text or strings.

☴ Overview:

  1. Pattern Matching
  2. Find and Replacing Text
  3. Extracting Text
  4. Validate Text
  5. Common Patterns

✦ Pattern Matching:

It used to match given or specific patterns within a string.

$string = "The sample text paragraph with some few words in the line.";
$pattern = "/sample text/";

if (preg_match($pattern, $string)) {
    echo "Match found!";
}
$pattern = "/\d{3}-\d{4}-\d{2}/";
$string = "Phone number: 123-4567-89.";

if (preg_match($pattern, $string, $matches)) {
    echo "Extracted Phone number: " . $matches[0];
} else {
    echo "No match found";
}

✦ Find and Replacing Text:

To find and replace the given text using regular expression.

$string = "The sample text paragraph with some few words in the line.";
$newString = preg_replace("/few/", "more", $string);
echo $newString; // Output: The sample text paragraph with some more words in the line.

✦ Extracting Text:

  • To extract specific part of the text from given string.
$string = "Kumara Vel (kumaravel@host.com)";
preg_match("/\((.*)\)/", $string, $matches);
echo $matches[1]; // Output: kumaravel@host.com
  • To match and extract all text from given string.
$string = "The sample text paragraph with sample few words in the line.";
preg_match_all("/sample/", $string, $matches);
print_r($matches);
  • To split text with given pattern.
$string = "apple, banana, orange";
$pattern = "/, /";
$fruits = preg_split($pattern, $string);
print_r($fruits);

✦ Validate Text:

To check or validate text in given format.

$email = "kumaravel@host.com";
if (preg_match("/^\w+@\w+\.\w+$/", $email)) {
    echo "Valid email address";
} else {
    echo "Invalid email address";
}

✦ Common Patterns:

  • Character Classes:
    • [abc]: Matches any single character from the set.
    • [^abc]: Matches any single character not in the set.
    • [a-z]: Matches any lowercase letter.
    • [A-Z]: Matches any uppercase letter.
    • [0-9]: Matches any digit.
  • Quantifiers:
    • +: Matches one or more repetitions of the preceding element.
    • *: Matches zero or more repetitions of the preceding element.
    • ?: Matches zero or one repetition of the preceding element.
    • {n}: Matches exactly n repetitions.
    • {n,}: Matches at least n repetitions.
    • {n,m}: Matches at least n and at most m repetitions.
  • Anchors:
    • ^: Matches the beginning of the string.
    • $: Matches the end of the string.
  • Grouping:
    • (): Groups a part of the pattern.
    • {}: Matches a specific number of occurrences of the preceding element.
    • []: Matches a character within the specified set.
  • Alternative:
    • |: Matches either the expression before or after the symbol.
  • Metacharacters:
    • . : Matches any single character except a newline.
    • \d: Matches a digit character.
    • \d{2}: Matches a two digit character.
    • \d+: Matches a multiple digit character.
    • \w: Matches any word character (alphanumeric or underscore).
    • \w{5}: Matches a five letter word.
    • \w+: Matches a multiple character word.
    • \W: Matches any non-word character.
    • \s: Matches any whitespace character.
    • \S: Matches any non-whitespace character.

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