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Checktestdata Regex

A regular expression (or regex) can be used to match strings. Formally, it describes a set of strings and a string is matched if it is contained in the set.

Regular expressions can contain both literal and special characters. Most literal characters, like A, a, or 0, are the simplest regular expressions and they simply match themselves. Additionally, more complex regular expressions can be expressed by concatenating simpler regular expressions. If A and B are both regular expressions, then AB is also a regular expression. In general, if a string x matches A and another string y matches B, then the string xy matches AB.

Besides the literal characters, there are also the following special characters: '(', ')', '{', '}', '[', ']', '*', '+', '?', '|', '\', '^', '.', '-'. Their meaning is as follows:

  • .: this matches any character, including newlines. If you need to match anything except the newline character use [^\n] instead.
  • []: indicates a set of characters. Inside a set definition:
    • Literal characters can be listed and all of them will be matched, i.e., [abc] will match 'a', 'b' as well as 'c' but not 'abc'.
    • Ranges can be specified with -, for example [a-z] will match any lowercase ASCII letter and [0-9] will match any digit. If - is escaped (e.g. [a\-z]) or if the character preceding it belongs to another range (e.g. [a-a-z]), or if it is the first or last character (e.g. [-a] or [a-]), it will match a literal '-'. It is an error if the first character of the range has a higher code point than the last (e.g., [z-a]).
    • The complement of a character set is formed if the first character of the set is ^. For example [^a] will match anything except 'a'. If ^ is escaped (e.g. [\^]) or if it is not the first character (e.g. [a^]) it will match a literal '^'.
    • \ can be used to escape a special characters. However, most special characters do not need to be escaped. Only '[' and ']' must be escaped and '^' or '-' might need to be escaped depending on the position. For example both [\-] and [-] will match a literal '-'. If \ is not followed by a special characters it matches a literal '\'.
    • It is an error if the character set does not specify any characters (e.g. [] or [^]).
  • {m,n}: causes the resulting regular expression to match from m to n repetitions of the preceding regular expression. Matching is done greedily, i.e., as many repetitions as possible are matched. Omitting m specifies a lower bound of zero, and omitting n specifies an infinite upper bound. It is an error if m is larger than n. Both m and n must be an integer without sign and without leading zeros. It is an error if the preceding regular expression is empty or ends with another repetition (e.g. {1,2}{1,2}). If you want to do that use () (e.g. ({1,2}){1,2}).
  • {m}: is a shorthand for {m,m}. It is an error to omit m.
  • *: is a shorthand for {0,}.
  • +: is a shorthand for {1,}.
  • ?: is a shorthand for {0,1}.
  • |: can be used to form the union of two regular expressions. If A and B are both regular expressions, then A|B is also a regular expression. In general, if a string x matches A or it matches B, then it also matches A|B. Matching is done in leftmost-first fashion. This means that any match of A is preferred over all matches for B. This means that the checktestdata command REGEX("p|ps") will only extract p even if the input is ps.
  • (...): if A is a regular expression then (A) is also a regular expression.
  • \: escapes the subsequent special character. If \ is not followed by a special character it will match a literal \ (e.g. \d will match '\d'). Note that checktestdata strings also use \ to escape characters. Therefore, REGEX("\\*") becomes the regular expression \* and matches a literal '*', not a variable amount of \.

Notes

The regular expression syntax and behaviour is carefully chosen to match a common subset of many modern regular expression definitions and implementations like Perl, Python, JavaScript, Ruby, PHP, Java, C++, Rust, Go, ... Advanced features like quantifiers, groups, lookahead, lookbehind, etc. are not supported. Shorthands like \d or [:digit:] are also not supported, use [0-9] instead.

Warning

Earlier versions of checktestdata used POSIX-like regular expressions with leftmost-longest matching and support for [:digit:]. This is no longer supported and matching is done leftmost-first instead.