Regex (php) to match single [ or single ] but ignore anything between [[ ]]?

Question

I have a string, and within it there will be combinations of [ ] [[ ]] ][ but I need to replace the single [ and ] with < and > but leave alone (don't match) anything that is between [[ ]].

I thought I could do this with a regex, but I'm really struggling to get it to work because the complexity is just beyond me at the moment.

Example string:

[a] [b]  <- should replace every [ with < and every ] with > so <a> <b>

[a][b]   <- should replace every [ with < and every ] with > so <a><b>

[[abc][a][b]]  <- should not replace anything. will always start with [[ and end with ]]

So thinking about this logically, I can do it in a loop with PHP but I really want to try and use a preg_replace if possible.

The logic, as far as I can decipher is to replace [ with < and ] with > EXCEPT between a [[ and ]] but I'm not sure if I can even do that in a regex. I can make it work partially by using lookahead/lookbehind but that still then matches [ and ] between [[ and ]] (e.g. [[ [a] ]].

So far I've got

    /(?<!(^|)\[)\[[^\]\[\[]*\]/gmi

Working to spot [a] but not [[a]] but fails if I have [[a [b] c]]. At this point I'm not worried about the replacement, I just need to get the regex working to match / not match.

g as a modifier won't work in PHP. Please provide code you are using. — user3783243
– user3783243, Commented Apr 11, 2023 at 13:00
In the future you should provide regex101 links so we can work with what you are working with. I think regex101.com/r/PjCOes/1 achieves your goal — user3783243
– user3783243, Commented Apr 11, 2023 at 13:19
Or this one: \[([^][]*)\]|\[(?:[^][]*(?R)?)*+\](*SKIP)(*F) — bobble bubble
– bobble bubble, Commented Apr 11, 2023 at 13:27
If you don't have mixed strings, a \G based idea: \G[^][]*\K\[([^][]*)\] (this is a bit more efficient and worth to be mentioned if you don't have strings like e.g. [[a][b]] [a][b]) — bobble bubble
– bobble bubble, Commented Apr 12, 2023 at 7:20

Wiktor Stribiżew · Accepted Answer · 2023-04-11 13:25:41Z

0

You can use

preg_replace('~(\[\[(?:(?!\[\[|]]).|(?1))*]])(*SKIP)(*F)|\[([^][]*)]~s', '<$2>', $text)

See the PHP demo and the regex demo.

Details:

(\[\[(?:(?!\[\[|]]).|(?1))*]])(*SKIP)(*F) - Group 1: [[, zero or more occurrences of any char that is not a starting point of the [[ or ]] char sequences or the whole Group 1 pattern recursed, and then ]], and once the match is found, it is skipped, the new search starts at the failure location
| - or
\[([^][]*)] - a [, then zero or more chars other than [ and ] captured into Group 2, and then a ].

answered Apr 11, 2023 at 13:25

Wiktor Stribiżew

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1 Comment

Richard Taylor Over a year ago

Thank you Wiktor, that is absolutely perfect and I now see how *SKIP works which I just couldn't get my head around.

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