正则表达式将匹配6个字符，只允许数字，前导和尾随空格

Question

The regex that I'm trying to implement should match the following data:

1. 123456
2. 12345
3. 23456
4. 5
5. 1
6.       
7. 2
8. 2345

It should not match the following:

1. 12 456
2. 1234 6
3. 1 6
4. 1 6

It should be 6 characters in total including the digits, leading, and trailing spaces. It could also be 6 characters of just spaces. If digits are used, there should be no space between them.

I have tried the following expressions to no avail:

^\s*[0-9]{6}$

\s*[0-9]\s*

Answer 1

You can just use a *\\d* * pattern with a restrictive (?=.{6}$) lookahead:

^(?=.{6}$) *\d* *$

See the regex demo

Explanation:

• ^ - start of string
• (?=.{6}$) - the string should only have 6 any characters other than a newline
• * - 0+ regular spaces ( NOTE to match horizontal space - use [^\\S\\r\\n] )
• \\d* - 0+ digits
• * - 0+ regular spaces
• $ - end of string.

Java demo (last 4 are the test cases that should fail):

List<String> strs = Arrays.asList("123456", "12345 ", " 23456", "     5", // good
"1     ", "      ", "  2   ", " 2345 ",  // good
"12 456", "1234 6", " 1   6", "1    6"); // bad
for (String str : strs)
    System.out.println(str.matches("(?=.{6}$) *\\d* *"));

Note that when used in String#matches() , you do not need the intial ^ and final $ anchors as the method requires a full string match by anchoring the pattern by default.

Answer 2

You can also do:

^(?!.*?\d +\d)[ \d]{6}$

• The zero width negative lookahead (?!.*?\\d +\\d) ensures that the lines having space(s) in between digits are not selected

• [ \\d]{6} matches the desired lines that have six characters having just space and/or digits.