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

Answer 1

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.

Answer 2

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).

Edit: added a ^ toward the beginning to catch the case of lines ending with */ followed by a new line. Thanks John Henry.

Answer 3

Try this:

^\s*$

Answer 4

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.

Answer 5

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.

Answer 6

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 '/^$/'.

Answer 7

Somehow none of the answers from here worked for me when I had strings which were filled just with spaces and occasionally strings having no content (just the line terminator), so I used this instead:

if (str.trim().isEmpty()) {
    doSomethingWhenWhiteSpace();
}

Answer 8

My use case was to replace the empty strings specifically in the Intellij-based IDE (Android Studio). For that one, this solution worked perfectly fine:

1. start replace ( command R )
2. select Regex replace
3. search for: ^\s*^
4. replace with: ( leave it empty )
5. click replace all.

explanation:

• ^ ( carrot / shift 6 in regex == start of line )
• \s* ( back slash s asterisk == any number of white space )
• ^\s*^ == start of line followed by any number of white space followed by start of line
• ^$ (carrot followed by dollar sign ) == line with nothing

Answer 9

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

Answer 10

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

\n\s*