Regular Expressions Cookbook

4.10. Limit the Number of Lines in Text

Problem

You need to check whether a string is comprised of five or fewer lines, without regard for how many total characters appear in the string.

Solution

The exact characters or character sequences used as line separators can vary depending on your operating system’s convention, application or user preferences, and so on. Crafting an ideal solution therefore raises questions about what conventions should be supported to indicate the start of a new line. The following solutions support the standard MS-DOS/Windows (‹\r\n›), legacy Mac OS (‹\r›), and Unix/Linux/OS X (‹\n›) line break conventions.

Regular expression

The following three flavor-specific regexes contain two differences. The first regex uses atomic groups, written as ‹(?>⋯)›, instead of noncapturing groups, written as ‹(?:⋯)›, because they have the potential to provide a minor efficiency improvement here for the regex flavors that support them. Python and JavaScript do not support atomic groups, so they are not used with those flavors. The other difference is the tokens used to assert position at the beginning and end of the string (‹\A› or ‹^› for the beginning of the string, and ‹\z›, ‹\Z›, or ‹$› for the end). The reasons for this variation are discussed in depth later in this recipe. All three flavor-specific regexes match exactly the same strings:

\A(?>(?>\r\n?|\n)?[^\r\n]*){0,5}\z

Regex options: None

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

\A(?:(?:\r\n?|\n)?[^\r\n]*){0,5}\Z

Regex ...