Question

This is a generalization of a specific problem I am trying to solve.

Given a string of uniquely identifiable elements separated by a single space:

0 1 2 3 4

Without modifying the string, what regular expression will match this expression when any of the numbers are optional except one and the numbers are in order?

Examples of other valid strings:

2
0 4
1 4
0 1 2 3
0 1 3 4
0 1 2 3 4

Answer 1

Regular expressions might not be the best tool for this. It’s just about manageable for the example you gave if you can use lookaheads:

^(?=0? ?1? ?2? ?3? ?4?$)(?:\d )*\d$

Rubular

Explanation:

^                       Match the start of line/string.
(?=0? ?1? ?2? ?3? ?4?$) Look ahead to restrict to 0 to 4 in that order.
(?:\d )*\d              Ensure that the string is alternating digit then space.
$                       Match the end of line/string.

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To generalize it to fixed length strings all of equal length makes it slightly more difficult to read. For example for ab ba cd de ef it will look like this:

^(?=(?:ab)? ?(?:ba)? ?(?:cd)? ?(?:de)? ?(?:ef)?$)(?:\w{2} )*\w{2}$

Rubular

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But generalizing it to words of varying length gets more messy, for example for zero one two three four you can use this regular expression:

^(?=(?:zero)? ?(?:one)? ?(?:two)? ?(?:three)? ?(?:four)?$)(?! )(?: ?(?:zero|one|two|three|four)(?= |$))+$

Rubular