regexp pattern for only allowing numbers at the end of a string?

I have a cell array of strings and need to figure out which strings match a certain pattern.

Rules for when the pattern should match

1. The string contains integers separated by '-'

2. The string has to start with '2' or '3'

3. The string has to end with '6'

4. The string has to end with an arbitrary repetition of '-5' (including 0 times) 

5. After the first occurence of '-5' no other numbers except the final 6 (see rule 3) are allowed (e.g. not allowed: 3-1-5-1-5-6)

Examples where the regexp pattern should match:

3-5-5-5-5-6

3-6

3-1-2-1-2-2-5-5-5-6

3-1-2-5-6

Examples where the regexp pattern should not match:

3-1-5-1-5-6

3-1-5-1-5-5-5-6

My question: Which regexp pattern realizes rules 1-4?

I've already managed to find a regext pattern that checks against rules 1-4 in the list above. However, I don't know how to add rule 5 to the pattern:

    % Create patternDirectRays = a regexp pattern for the direct rays     
    %     Example: The regexp pattern <pattern> for start numbers 2 and 3, last-but-one-numbers 5 and last number 6 is
    %        pattern = '^(2|3)(-5)*(-6)$'  => ^(2|3) means 'has to start with 2 or 3' / (-5)* means ...followed by 0,1 or more repetitions of '-5' / (-6) = ..and finishes with '-6'
    %     The following strings would match <pattern>:
    %        regexp('3-5-5-6',     '^(2|3)(-5)*(-6)$', 'match')   => '3-5-5-6'
    %        regexp('2-5-5-6',     '^(2|3)(-5)*(-6)$', 'match')   => '3-5-5-6'
    %        regexp('3-5-5-5-5-6', '^(2|3)(-5)*(-6)$', 'match')   => '3-5-5-5-5-6'
    %        regexp('3-6',         '^(2|3)(-5)*(-6)$', 'match')   => '3-6'
    myPattern = '^(2|3)(-5)*(-6)$'

    % Get indices of matching strings
    idx = regexp(rayPaths, myPattern, 'start');

Answer 1

[Anushka]

Hello @Wolf,

To include Rule 5 in your regular expression pattern, you need to ensure that after the first occurrence of -5, no other numbers except the final -6 are allowed. Follow the below mentioned steps to construct the pattern :

1. Start with 2 or 3.

2. Allow any sequence of numbers and dashes, but after the first occurence of -5 , only -5 or -6 can follow.

3. Ensure the string ends with -6.

Here's the updated regular expression pattern:

myPattern = '^(2|3)(-[0-9]+)*(-5)*-6$';

For applying the pattern, 'regexp' function can be utilized.

Refer to the below mentioned MATLAB Documentation link for the ‘regexp’ function:

https://www.mathworks.com/help/matlab/ref/regexp.html

Hope this helps.