A case study of regular expression usage in batch processing findstr _dos/bat

1.findstr. 2.txt or Findstr "." 2.txt
Find any character from file 2.txt, excluding null characters or blank lines

2.findstr. * 2.txt or findstr ". *" 2.txt
Find any character from file 2.txt including blank lines and null characters

3.findstr "[0-9]" 2.txt
Find a string or row that includes the number 0-9 from file 2.txt

4.findstr "[A-za-z]" 2.txt
Find a string or row that includes any character from file 2.txt

5.findstr "[Abcezy]" 2.txt
Find a string or line that includes a b c e z y letter from File 2.txt

6.findstr "[A-fl-z]" 2.txt
Finds the lowercase character a-f l-z string from file 2.txt, but does not contain the letters G H I J K.

7.findstr "M[abc][hig]y" 2.txt
From the file 2.txt can match Mahy, Mbiy, Mahy, etc. .....

8. ^ and $ application of symbols
^ Represents the beginning of the line, and "^step" matches only the first word in "Step Hello World"
$ indicates line end, "step$" matches only the last word in "Hello World"

9.finstr "[^0-9]" 2.txt
If it is a string of pure numbers or a line filtered out, such as 2323423423 string, if it is 345hh888 such a form will not be.

10.findstr "[^a-z]" 2.txt
Ditto, if the string or line of pure letters filtered out, such as sdlfjlkjlksjdklfjlskdf characters, if it is sdfksjdkf99999 such a form, doped with numbers will not be

The role of 11.* number
As mentioned earlier, ". *" means that the search condition is any character, the * number is not in the regular expression of any character, but the left character or the number of repetitions of the expression, * the number of repetitions is 0 or more times.

12.findstr "^[0-9]*$" 2.txt
This is a match to find the pure number, such as 234234234234, if it is 2133234kkjl234 is filtered out.
Findstr "^[a-z]*$" 2.txt
This is a match to find the pure letter, such as SDFSDFSDFSDF, if it is 213sldjfkljsdlk is filtered out
If there are no * numbers in the search terms, that is, do not repeat the search conditions on the left, that is, [0-9] [A-z] that can only match the first character of the string and only this one character, because there is the beginning and end of the limit, "^[0-9]$" the first character if the number is matched, if not filtered out, If the string is 9 match, if it is 98 or 9j and so on is not OK.

The role of the expression "\<...\>"
This representation precisely looks for a string, \<sss the beginning position of the word,sss\> the end position of the word
echo Hello World computer|findstr "\<computer\>" in this form
echo Hello worldcomputer|findstr "\<computer\>" is not the form, he is looking for "computer" this string, so not.
echo Hello worldcomputer|findstr ".*computer\>" so you can match it.