Regular Expression Matching Repeating Patterns

Question

I'm trying to develop regular expressions to check syntax for question writing for a quiz module.

One question type is drag and drop which has the format ...

the {rain} falls gently on the {plain}

... where the bracketed terms are draggable. There has to be at least one bracketed term but they can come anywhere in the string. The doesn't need to be anything between the bracketed terms either. So any of the following are valid ...

{the} rain falls gently on the plain

the rain falls gently on the {plain}

{the} {rain} {falls} {gently} {on} {the} {plain}

{the }{rain }{falls }{gently }{on }{the }{plain}

Also, to complicate matters, there can't be any square brackets inside the braces - this is another question type. So ...

{the} {rain}

... is OK, but ...

{the} {[rain]}

... is not and any other combination.

So far, I've got this but it clearly doesn't work :(

(.*\{.*\})+

Please can someone help me out?

This may do the trick regex101.com/r/k2BTpu/2

Eraklon
– Eraklon

2020-02-10 21:11:12 +00:00
Commented Feb 10, 2020 at 21:11 — Eraklon
– Eraklon, Commented Feb 10, 2020 at 21:11
Can there be { and } between the {}?

The fourth bird
– The fourth bird

2020-02-10 21:24:24 +00:00
Commented Feb 10, 2020 at 21:24 — The fourth bird
– The fourth bird, Commented Feb 10, 2020 at 21:24
Yes it very well can be.

Eraklon
– Eraklon

2020-02-10 21:27:58 +00:00
Commented Feb 10, 2020 at 21:27 — Eraklon
– Eraklon, Commented Feb 10, 2020 at 21:27

The fourth bird · Accepted Answer · 2020-02-10 21:40:53Z

1

If { and } can not occur between the {} and the square brackets can be present outside of the curly braces but not inside, you might use:

^[^\r\n{}]*(?:{[^\][\r\n{}]*}[^\r\n{}]*)+$

^ Start of string
[^\r\n{}]* Repeat 0+ times any char except a newline or { and }
(?: Non capture group
- {[^\][\r\n{}]*} Match {, then 0+ times any char except a newline or any of []{}
- [^\r\n{}]* Match 0+ times any following chars except a newline or { and }
)+ Close group and repeat at least 1 or more times to make sure there is a draggable {...}
$ End of string

Regex demo

edited Feb 10, 2020 at 21:40

answered Feb 10, 2020 at 21:32

The fourth bird

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9 Comments

MrMills Over a year ago

Yep - thank you - that seems to work perfectly. I shall study it and learn more :)

MrMills Over a year ago

After studying this a little, do you never need to specify "any character" as a dot at any point in this? So is matching any character explicit in RegEx? Sorry for not getting this :(

The fourth bird Over a year ago

It is not mandatory to use a dot to write a pattern. In this pattern the negated character classes specify what you want to allow to match except what is listed.

MrMills Over a year ago

Right - so this could be the issue with this - I can't get the string to match without the .+ being present but then it matches everything after it, even if the syntax is wrong. regex101.com/r/oHz9P8/1 I guess I need to use capturing groups? The correct syntax must be ... {[answer][yes]}{[answer][no]}{[answer][no]}{[answer][no]} There can only be one 'yes' and as many 'no' as required (there is no limit on options in the question) and the 'yes' and 'no's can be anywhere in the string. Phew - this is harder than I thought.

The fourth bird Over a year ago

@MrMills The .+ is a very broad match and will match until the end of the string first and will then backtrack trying to fit the rest of the pattern. You might do it like this by repeating the no patterns and match at least a single yes pattern regex101.com/r/JKJiYb/1

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