Regular Expressions Cookbook, 2nd Edition

2.5. Match Something at the Start and/or the End of a Line

Problem

Create four regular expressions. Match the word alpha, but only if it occurs at the very beginning of the subject text. Match the word omega, but only if it occurs at the very end of the subject text. Match the word begin, but only if it occurs at the beginning of a line. Match the word end, but only if it occurs at the end of a line.

Solution

Start of the subject

^alpha

Regex options: None (“^ and $ match at line breaks” must not be set)

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python

\Aalpha

Regex options: None

Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby

End of the subject

omega$

Regex options: None (“^ and $ match at line breaks” must not be set)

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python

omega\Z

Regex options: None

Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby

Start of a line

^begin

Regex options: ^ and $ match at line breaks

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

End of a line

end$

Regex options: ^ and $ match at line breaks

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Discussion

Anchors and lines

The regular expression tokens ‹^›, ‹$›, ‹\A›, ‹\Z›, and ‹\z› are called anchors. They do not match any characters. Instead, they match at certain positions, effectively anchoring the regular expression match at those positions.

A line is the part of the subject text that lies between the start of the subject and a line break, between two line breaks, or between ...