How to check if a line is blank using regex

Question

I am trying to make simple regex that will check if a line is blank or not.

Case;

"    some"   // not blank
"   " //blank
"" // blank

Here Blank mean what you are meaning. A line contains full of whitespaces or a line contains nothing. If you want to match a line which contains nothing then use '/^$/' — Badri Gs, Aug 04 '17 at 05:05

score 418 · Accepted Answer · edited Feb 10 '17 at 15:12

418

The pattern you want is something like this in multiline mode:

^\s*$

Explanation:

^ is the beginning of string anchor.
$ is the end of string anchor.
\s is the whitespace character class.
* is zero-or-more repetition of.

In multiline mode, ^ and $ also match the beginning and end of the line.

References:

regular-expressions.info/Anchors, Character Classes, and Repetition.

A non-regex alternative:

You can also check if a given string line is "blank" (i.e. containing only whitespaces) by trim()-ing it, then checking if the resulting string isEmpty().

In Java, this would be something like this:

if (line.trim().isEmpty()) {
    // line is "blank"
}

The regex solution can also be simplified without anchors (because of how matches is defined in Java) as follows:

if (line.matches("\\s*")) {
    // line is "blank"
}

API references

String String.trim()
- Returns a copy of the string, with leading and trailing whitespace omitted.
boolean String.isEmpty()
- Returns true if, and only if, length() is 0.
boolean String.matches(String regex)
- Tells whether or not this (entire) string matches the given regular expression.

edited Feb 10 '17 at 15:12

Elliot Labs LLC

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answered Jun 10 '10 at 08:35

polygenelubricants

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@Adnan: take note of Bart's comment in Marcelo's answer; depending on how you want to handle multiple blank lines, the pattern may change slightly. – polygenelubricants Jun 10 '10 at 08:37
Well I am reading a file from Java, line by line, so I assume that this will be ok. – Adnan Jun 10 '10 at 08:42
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that seems brilliant with line.trim :D – Adnan Jun 10 '10 at 08:54
1

excellent the codes now executes from 1.6sec to >1sec Thank you. – Adnan Jun 10 '10 at 08:57
in python: `if line.strip():` since an empty string evaluates to `False` – John Mark Oct 28 '19 at 21:22
This is incorrect. When the line end is "\r\n" it finds a single line end, not the blank line "\r\n\r\n". – mirik Mar 22 '21 at 18:40

score 75 · Answer 2 · answered Sep 01 '16 at 23:45

75

Actually in multiline mode a more correct answer is this:

/((\r\n|\n|\r)$)|(^(\r\n|\n|\r))|^\s*$/gm

The accepted answer: ^\s*$ does not match a scenario when the last line is blank (in multiline mode).

answered Sep 01 '16 at 23:45

bchr02

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Exactly, and I confirmed this is the case. The accepted answer missed many empty lines in my file, but this caught them all. The union of both regexes catches every case. – elmor Oct 05 '17 at 00:16
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This answer worked perfectly in a tool such as Notepad++. The accepted answer matched multiple empty lines but not single empty lines. – james Nov 02 '17 at 17:54
In my case, the accepted answer not work in Sublime Text 3, and this works. – yangsibai Feb 07 '18 at 03:20
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The accepted answer did work for my case, multilines. This does. – Robert Gabriel Mar 23 '18 at 11:02
1

this should be the best answer. – Q i Jul 22 '19 at 23:46
the chosed one is not for delete empty lines. this does. so for me this one is best as an answer. – Zen Of Kursat Dec 06 '19 at 08:53
There is a small fix on top of this in [this other answer](https://stackoverflow.com/a/46660616/11154841) (if you have `*/` (end of comment) followed by an empty line). – questionto42standswithUkraine Jan 30 '22 at 15:11

score 14 · Answer 3 · answered Jun 10 '10 at 08:32

14

Try this:

^\s*$

answered Jun 10 '10 at 08:32

Marcelo Cantos

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@Adnan, note that `\s` also matches line breaks, so you won't "find" single empty lines inside a string containing successive empty lines. – Bart Kiers Jun 10 '10 at 08:34

score 9 · Answer 4 · answered Oct 10 '17 at 07:12

9

Full credit to bchr02 for this answer. However, I had to modify it a bit to catch the scenario for lines that have */ (end of comment) followed by an empty line. The regex was matching the non empty line with */.

New: (^(\r\n|\n|\r)$)|(^(\r\n|\n|\r))|^\s*$/gm

All I did is add ^ as second character to signify the start of line.

answered Oct 10 '17 at 07:12

John Henry

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Why not comment on bchr02's answer and suggest the improvement to his answer? – adamlogan Jun 21 '18 at 08:23
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@adamlogan yeah, from memory, I think I wanted to do it at the time, but I didn't have enough reputation to comment on someone else's post, so had to do it like this. – John Henry Jun 27 '18 at 01:41

score 6 · Answer 5 · answered Jun 10 '10 at 08:34

6

The most portable regex would be ^[ \t\n]*$ to match an empty string (note that you would need to replace \t and \n with tab and newline accordingly) and [^ \n\t] to match a non-whitespace string.

answered Jun 10 '10 at 08:34

soulmerge

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I'd at least change the single space with the class `[ \t]` – Bart Kiers Jun 10 '10 at 08:35
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On Windows you also need to consider the carriage return character `\r` so the regex would be `^[ \t\r\n]*$`. But `^\s*$` is better - more concise. If you don't want to match newlines, you can use `\h` (meaning horizontal whitespace) as in `^\h*$` – ps.pf Sep 07 '15 at 06:03

score 3 · Answer 6 · answered Jun 10 '10 at 08:37

3

Here Blank mean what you are meaning.
A line contains full of whitespaces or a line contains nothing.
If you want to match a line which contains nothing then use '/^$/'.

answered Jun 10 '10 at 08:37

kiruthika

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score -1 · Answer 7 · answered Aug 31 '17 at 08:31

-1

Well...I tinkered around (using notepadd++) and this is the solution I found

\n\s

\n for end of line (where you start matching) -- the caret would not be of help in my case as the beginning of the row is a string \s takes any space till the next string

hope it helps

answered Aug 31 '17 at 08:31

M_TRONIC

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OP wants a regex answer, which was given, and is not about new line characters. – moodymudskipper Aug 31 '17 at 08:54
user asks for a "simple regex that will check if a line is blank" this regex (tested in regexpal.com) does exactly that. why dont u test it? – M_TRONIC Aug 31 '17 at 09:53
using R, our test vector: `test_vec – moodymudskipper Aug 31 '17 at 10:36
like I said i used this on notepad++ and it worked. yours didn't . so i guess we can agree to disagree! – M_TRONIC Aug 31 '17 at 11:01
I'm not really sure what you're doing though, are you doing `ctrl+f` in notepad++ ? In this case you can find (though not really match) the blank lines by selecting "Extended" Search mode and searching for '\n\s', if you select "Regular Expression', your string will match the same, and you can also try @polygenelubricants 's solution. The latter will really match the line, you can check and see the difference. I would suggest that you edit your answer to be more clear about what you're advising, so readers can take more value from it. – moodymudskipper Aug 31 '17 at 11:15

Just Me · Answer 8 · 2022-04-04T18:45:54.100

-1

This regex will delete all empty spaces (blank) and empty lines and empty tabs from file

\n\s*

edited Apr 04 '22 at 18:45

answered Apr 04 '22 at 14:16

Just Me

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As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the [help center](https://stackoverflow.com/help/how-to-answer). – Tyler2P Apr 04 '22 at 17:05
This didn't work for me. It also finds end-of-lines. – Gabriel Jun 01 '22 at 19:36

How to check if a line is blank using regex

8 Answers8

References:

A non-regex alternative:

API references

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