Disallow character if not prefixed with another character using regex

Question

I have incorporated the following regular expression into my code:

.gsub(/(^|[^*])(\*\*)([^*]+)(\*\*)($|[^*])/m, '\1*\3*\5') # bold

The issue I'm facing is that the \3 block (defined as [^*]+) doesn't currently permit asterisks * within the text.

My objective is to modify the regular expression in such a way that it allows asterisks only if they are preceded by an escape character \. What is the appropriate approach to achieve this modification?

Examples: **hello \* world** I want it to output *hello \* world*

But I got the text with no change with the current regex.

If you are dealing with Markdown, it's best to use a parser. Potential duplicates: Ruby: how to generate HTML from Markdown like GitHub's or BitBucket's?, Better ruby markdown interpreter?. — InSync
– InSync, Commented Oct 10, 2023 at 16:42
@OmarFareed can you explain what other thing you are doing and why a Markdown parser doesn't help? — Stefan
– Stefan, Commented Oct 11, 2023 at 6:32

Wiktor Stribiżew · Accepted Answer · 2023-10-10 16:43:25Z

1

In these situations, you can use an unrolled [^*\\]*(?:\\.[^*\\]*)*:

.gsub(/(^|[^*])(\*\*)([^*\\]*(?:\\.[^*\\]*)*)(\*\*)($|[^*])/m, '\1*\3*\5') # bold

See the regex demo. Which can be further optimized to

.gsub(/(?<!\*)\*\*([^*\\]*(?:\\.[^*\\]*)*)\*\*(?!\*)/m, '*\1*')

See this regex demo

Details:

(?<!\*) - a position that is not immediately preceded with a *
\*\* - a ** string
([^*\\]*(?:\\.[^*\\]*)*) - Group 1: any zero or more chars other than \ and * and then zero or more sequences of a \ char followed by any other char and then zero or more characters other than * and \
\*\* - a ** string
(?!\*) - a position that is not immediately followed with a *

See the Ruby demo:

text = '**hello \* world**'
puts text.gsub(/(?<!\*)\*\*([^*\\]*(?:\\.[^*\\]*)*)\*\*(?!\*)/m, '*\1*') 
# => *hello \* world*

edited Oct 10, 2023 at 16:43

answered Oct 10, 2023 at 16:31

Wiktor Stribiżew

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7 Comments

Omar Fareed Over a year ago

i didn't get the expected output with this regex, i added an example if the question was not clear.

Wiktor Stribiżew Over a year ago

Also, consider gsub(/(?<!\*)((?:\*\*)*)\*\*([^*\\]*(?:\\.[^*\\]*)*)\*\*((?:\*\*)*)(?!\*)/, '\1*\2*\3') for more complex scenarios. The \ns in the demo are just for line-by-line demo.

Omar Fareed Over a year ago

and thanks for the demo, I will have a look at it

Wiktor Stribiżew Over a year ago

@OmarFareed That does exactly what you asked for. See regex101.com/r/gDCCI6/6. I also added a Ruby demo.

Wiktor Stribiżew Over a year ago

@OmarFareed See What is a non-capturing group in regular expressions? and Regex lookahead, lookbehind and atomic groups. Besides, see regular-expressions.info and rexegg.com.

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