regular expression notation and meaning.
egular expression all 20 notation and example string or line? example \w ?
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20 Common Regular Expression Notations and Examples
Basic Matching:
. (Dot): Matches any single character except a newline.
Example: r".at" matches "cat", "hat", "bat", etc.
[] (Character Class): Matches a single character from the specified set.
Example: r"[abc]at" matches "cat", "bat", or "at".
[^...] (Negated Character Class): Matches a single character not in the specified set.
Example: r"[^aeiou]" matches any consonant.
Quantifiers:
* (Star): Matches zero or more repetitions of the preceding element.
Example: r"ha*" matches "h", "ha", "haa", etc.
+ (Plus): Matches one or more repetitions of the preceding element.
Example: r"ha+" matches "ha", "haa", "haaa", etc.
? (Question Mark): Matches zero or one repetition of the preceding element.
Example: r"colou?r" matches "color" or "colour".
{m,n} (Range Quantifier): Matches at least m and at most n repetitions.
Example: r"a{2,4}b" matches "aab", "aaab", or "aaaab".
Anchors:
^ (Caret): Matches the beginning of a line.
Example: r"^The" matches lines starting with "The".
$ (Dollar): Matches the end of a line.
Example: r"world$" matches lines ending with "world".
Special Sequences:
\d: Matches a digit character.
Example: r"\d{3}-\d{2}-\d{4}" matches phone numbers like "123-45-6789".
\D: Matches a non-digit character.
Example: r"\D+" matches any non-digit sequence.
\s: Matches a whitespace character (space, tab, newline, etc.).
Example: r"\s+" matches multiple whitespace characters.
\S: Matches a non-whitespace character.
Example: r"\S+" matches any non-whitespace sequence.
\w: Matches an alphanumeric character (letter, number, or underscore).
Example: r"\w+" matches words.
\W: Matches a non-alphanumeric character.
Example: r"\W+" matches punctuation marks and other symbols.
Grouping and Alternation:
() (Grouping): Groups parts of a regular expression.
Example: r"(ab)+" matches "ab", "abab", "ababab", etc.
| (Alternation): Matches either the expression before or after it.
Example: r"cat|dog" matches "cat" or "dog".
Lookaround Assertions:
(?=...) (Positive Lookahead): Asserts that the following expression matches, but doesn't consume characters.
Example: r"\w+(?=\s)" matches words followed by whitespace.
(?!...) (Negative Lookahead): Asserts that the following expression doesn't match.
Example: r"\w+(?!\d)" matches words not followed by a digit.
(?<=...) (Positive Lookbehind): Asserts that the preceding expression matches.
Example: r"(?<=^)The" matches "The" at the beginning of a line.
Remember:
Regular expressions can be complex and powerful.
Use online regex testers or debuggers to experiment and visualize matches.
Consider using libraries like re in Python for more advanced regex operations.
By understanding these notations and practicing, you can effectively use regular expressions to solve a variety of text processing tasks.
next 10 examples?
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Sure, here are 10 more examples of regular expressions and how they can be used to match text patterns:
Pattern Description Example Match
\b\w+\b Matches a single word This is a sentence.
\w+\s+\w+ Matches two words separated by a space This is a great example.
\d+-\d+-\d+ Matches a date in the format YYYY-MM-DD The meeting is on 2023-11-21.
[A-Z][a-z]+ Matches a word that starts with a capital letter and is followed by lowercase letters The cat is on the mat.
\$\d+\.\d{2} Matches a currency value in US dollars format The cost is $123.45.
[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-zA-Z]+ Matches a basic email address My email is user@example.com.
www\.[a-zA-Z0-9]+\.[a-zA-Z]+ Matches a simple domain name The website is www.google.com.
\([0-9]{3}\) [0-9]{3}-\d{4} Matches a phone number in (XXX) XXX-XXXX format My phone number is (123) 456-7890.
\color{.*?} Matches HTML color code The text is <color=red>red</color>.
<img src=".*?" alt=".*?"> Matches a basic HTML image tag The image is <img src="image.jpg" alt="My image">