Regular Expressions Cookbook, 2nd Edition

Keywords

Problem

You are working with a file format for forms in a software application. The words “end,” “in,” “inline,” “inherited,” “item,” and “object” are reserved keywords in this format.^[9] You want a regular expression that matches any of these keywords.

Solution

The basic solution is very straightforward and works with all regex flavors in this book:

\b(?:end|in|inline|inherited|item|object)\b

Regex options: Case insensitive

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

We can optimize the regular expression for regex flavors that support atomic grouping:

\b(?>end|in(?:line|herited)?|item|object)\b

Regex options: Case insensitive

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

Discussion

Matching a word from a list of words is very easy with a regular expression. We simply use alternation to match any one of the keywords. The word boundaries at the start and the end of the regex make sure we only match entire words. The regex should match inline rather than in when the file contains inline, and it should fail to match when the file contains interesting. Because alternation has the lowest precedence of all regex operators, we have to put the list of keywords inside a group. Here we used a noncapturing group for efficiency. When using this regex as part of a larger regular expression, you may want to use a capturing group instead, so you can determine whether the regex matched a keyword or something else.

We can optimize this regular expression when using regular ...