Regex Syntax and Basic Patterns
Regex Syntax: Building Blocks
Regular expressions, or regex, are constructed using a combination of literals, metacharacters, and character classes. Understanding these components is essential for writing effective patterns.
- Literals are ordinary characters that match themselves. For example, the pattern
"cat"matches the exact string"cat"; - Metacharacters are special symbols that have unique meanings in regex. The most common metacharacters include:
.(dot): Matches any single character except a newline;^: Matches the start of a string;$: Matches the end of a string;*: Matches zero or more occurrences of the preceding element;+: Matches one or more occurrences of the preceding element;?: Matches zero or one occurrence of the preceding element.
- Character classes allow you to match any one character from a set of characters. For example,
[abc]matches"a","b", or"c", and[0-9]matches any digit from 0 to 9.
123456789101112131415import re text = "User123, admin42, guestX" # Find all digits digits = re.findall(r"[0-9]", text) print("Digits found:", digits) # Find all lowercase letters lowercase_letters = re.findall(r"[a-z]", text) print("Lowercase letters found:", lowercase_letters) # Find all uppercase letters uppercase_letters = re.findall(r"[A-Z]", text) print("Uppercase letters found:", uppercase_letters)
In this example, you use the re.findall() function to search for matches in the string "User123, admin42, guestX". The pattern [0-9] matches any digit between 0 and 9, so it finds all individual digits in the text. The pattern [a-z] matches any lowercase letter from "a" to "z", and [A-Z] matches any uppercase letter from "A" to "Z". Each character class is enclosed in square brackets, specifying the range or set of characters to match. These patterns are fundamental in extracting specific types of characters from text.
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Regex Syntax: Building Blocks
Regular expressions, or regex, are constructed using a combination of literals, metacharacters, and character classes. Understanding these components is essential for writing effective patterns.
- Literals are ordinary characters that match themselves. For example, the pattern
"cat"matches the exact string"cat"; - Metacharacters are special symbols that have unique meanings in regex. The most common metacharacters include:
.(dot): Matches any single character except a newline;^: Matches the start of a string;$: Matches the end of a string;*: Matches zero or more occurrences of the preceding element;+: Matches one or more occurrences of the preceding element;?: Matches zero or one occurrence of the preceding element.
- Character classes allow you to match any one character from a set of characters. For example,
[abc]matches"a","b", or"c", and[0-9]matches any digit from 0 to 9.
123456789101112131415import re text = "User123, admin42, guestX" # Find all digits digits = re.findall(r"[0-9]", text) print("Digits found:", digits) # Find all lowercase letters lowercase_letters = re.findall(r"[a-z]", text) print("Lowercase letters found:", lowercase_letters) # Find all uppercase letters uppercase_letters = re.findall(r"[A-Z]", text) print("Uppercase letters found:", uppercase_letters)
In this example, you use the re.findall() function to search for matches in the string "User123, admin42, guestX". The pattern [0-9] matches any digit between 0 and 9, so it finds all individual digits in the text. The pattern [a-z] matches any lowercase letter from "a" to "z", and [A-Z] matches any uppercase letter from "A" to "Z". Each character class is enclosed in square brackets, specifying the range or set of characters to match. These patterns are fundamental in extracting specific types of characters from text.
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