Introduction to Regular Expressions
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Regular expressions (regex) are a language used to parse text and apply logic and constraints to find patterns in strings. They provide a concise way to find matches in text that is supported across many programming languages. This document provides an overview of regex, examples of code using regex in different languages, and descriptions of common regex patterns and metacharacters used to define matching rules. It recommends resources for further reading on mastering regular expressions.
![Character Classes
POSIXPOSIX ShorthandShorthand LonghandLonghand DescriptionDescription
[:word:] w [A-Za-z0-9_] Alphanumeric Chars.
W [^A-Za-z0-9_]
Non-alphanumeric
Chars.
[:alpha:] [A-Za-z] Alphabetic Chars.
[:blank:] [ t] Space and tab
[:digit:] d [0-9] Numeric Characters
D [^0-9] Non-numeric Chars.
[:space:] s [ trnvf] Whitespace Characters](https://image.slidesharecdn.com/regularexpressions-101107185852-phpapp02/85/Introduction-to-Regular-Expressions-10-320.jpg)
![Groups
Source TextSource Text Regular ExpressionRegular Expression YieldYield
“hello world” “([a-z]+)s+([a-z]+)
{ “hello world”,
“hello”, “world” }
“hello world12345” “([a-z]+)s+([a-z]+)
{ “hello world”,
“hello”, “world” }
“hello world12345” “([a-z]+)s+([a-z]+)(d+)
{ “hello
world12345”, “hello”,
“world”, “12345” }](https://image.slidesharecdn.com/regularexpressions-101107185852-phpapp02/85/Introduction-to-Regular-Expressions-11-320.jpg)
Introduction to Regular Expressions
- 2. What Are They? • Language to parse text • Apply logic and constraints • Concise (but not readable) • Consistent (mostly) • Widely supported in programming languages
- 3. Hello Regex Source Text Regular Expression Yield “hello world” “hello” { “hello” } “hello world hello world” “hello” { “hello”, “hello” } “hello world hello world” “world” { “world”, “world” } “hello world hello world” “hello world” { “hello world”, “hello world” }
- 4. Java Regex Code Pattern pattern = Pattern.compile("hello"); Matcher matcher = pattern.matcher("hello world"); // Find all matches while (matcher.find()) { // Get the matching string String match = matcher.group(); // match = “hello” }
- 5. C# Regex Code foreach (Match match in Regex.Matches("hello world", "hello", RegexOptions.IgnoreCase)) { // Get the matching string String match = match.Value; // match = “hello” }
- 6. Python Regex Code regex = re.compile("hello"); results = regex.search("hello world"); // results = "hello"
- 7. Perl Regex Code $value = "hello world"; $value =~ m/hello/; $result = $1; // result = "hello"
- 8. The (Ugly) Alternative String needle = "hello"; String haystack = "hello world hello world"; int index = 0; while ((index = haystack.indexOf( needle, index )) != -1) { String match = haystack.substring( index, index + needle.length() ); index++; }
- 9. Regex Metacharacters • * - Match zero or more times • ? - Match zero or 1 time • + - Match one or more times • ^ - Match the start of a string • $ - Match the end of a string
- 10. Character Classes POSIXPOSIX ShorthandShorthand LonghandLonghand DescriptionDescription [:word:] w [A-Za-z0-9_] Alphanumeric Chars. W [^A-Za-z0-9_] Non-alphanumeric Chars. [:alpha:] [A-Za-z] Alphabetic Chars. [:blank:] [ t] Space and tab [:digit:] d [0-9] Numeric Characters D [^0-9] Non-numeric Chars. [:space:] s [ trnvf] Whitespace Characters
- 11. Groups Source TextSource Text Regular ExpressionRegular Expression YieldYield “hello world” “([a-z]+)s+([a-z]+) { “hello world”, “hello”, “world” } “hello world12345” “([a-z]+)s+([a-z]+) { “hello world”, “hello”, “world” } “hello world12345” “([a-z]+)s+([a-z]+)(d+) { “hello world12345”, “hello”, “world”, “12345” }
- 12. Example • Example that parses, cleans up, and normalizes input
- 13. Recommended Reading • Mastering Regular Expressions by Jeffry Friedl • Regular Expressions Cheat Sheet http://www.addedbytes.com/cheat-sheets/regular-e • Regex Evaluator http://www.cuneytyilmaz.com/prog/jrx/