.Net regex matching $ with the end of the string and not of line, even with multiline enabled

Question

I'm trying to highlight markdown code, but am running into this weird behavior of the .NET regex multiline option.

The following expression: ^(#+).+$ works fine on any online regex testing tool:

But it refuses to work with .net:

It doesn't seem to take into account the $ tag, and just highlights everything until the end of the string, no matter what. This is my C#

RegExpression = new Regex(@"^(#+).+$", RegexOptions.Multiline)

What am I missing?

A dot cannot match a LF without a DOTALL modifier. Can you please share a dotnetfidle? — Wiktor Stribiżew, Oct 15 '16 at 11:44

Wiktor Stribiżew · Accepted Answer · 2018-07-02T10:29:36.720

6

It is clear your text contains a linebreak other than LF. In .NET regex, a dot matches any char but LF (a newline char, \n).

See Multiline Mode MSDN regex reference

By default, $ matches only the end of the input string. If you specify the RegexOptions.Multiline option, it matches either the newline character (\n) or the end of the input string. It does not, however, match the carriage return/line feed character combination. To successfully match them, use the subexpression \r?$ instead of just $.

So, use

@"^(#+).+?\r?$"

The .+?\r?$ will match lazily any one or more chars other than LF up to the first CR (that is optional) right before a newline.

Or just use a negated character class:

@"^(#+)[^\r\n]+"

The [^\r\n]+ will match one or more chars other than CR/LF.

edited Jul 02 '18 at 10:29

answered Oct 15 '16 at 13:12

Wiktor Stribiżew

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Unfortunately when I use `Regex.Replacec` it replaces and `\r ` character (( Can I fix it? – Denis535 Sep 14 '19 at 01:44
@Deni35 It seems you have a new question, please ask – Wiktor Stribiżew Sep 14 '19 at 08:53
One could match end of line with this pattern: `(?=\r?$)` – Denis535 Sep 16 '19 at 15:55

score 0 · Answer 2 · answered Oct 15 '16 at 11:51

0

What you have is good. The only thing you're missing is that . doesn't match newline characters, even with the multiline option. You can get around this in two different ways.

The easiest is to use the RegexOptions.Singleline flag which cause newlines to be treated as characters. That way, ^ still matches the start of the string, $ matches the end of the string and . matches everything including newlines.

The other way to fix this (although I wouldn't recomend it for your use case) is to modify your regex to explicitly allow newlines. To do this you can just replace any . with (?:.|\n) which means either anycharacter or a newline. For your example, you would end up with ^(#+)(?:.|\n)+$. If you want to ensure that there's a non-linebreak character first, add an extra dot: ^(#+).(?:.|\n)+$

answered Oct 15 '16 at 11:51

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I think you've misunderstood my question. I don't want to match new lines. The second image is what I've got, and the first image is what I SHOULD get. The input string "this is a \n #header \n but this isn't" should only match "#header". Currently, it's matching "#header but this isn't" – user2950509 Oct 15 '16 at 12:43
Please never suggest `(?:.|\n)+` pattern. It is very inefficient and can cause system freeze because of the number of backtracking (or expansion in case of lazy quantifier) steps it has to perform. Always use `.` with the `(?s)` inline modifier or `RegexOptions.Singleline` in .NET. You do not need to every use the `[\s\S]` like workarounds as you may use modifier groups in .NET regex. e.g.: `^.*\r?\n(?s:.*)`. – Wiktor Stribiżew Jul 02 '18 at 10:32

.Net regex matching $ with the end of the string and not of line, even with multiline enabled

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