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.

1. Regular
Expressions
Jesse Anderson

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/