Check for only numerical input in batch file

Edited to fix the regex as per debham's comment. Turns out that adding a space before the pipe after echo adds a space to the piped string, which was what broke the start/end of line matching previously. The regex could be further improved by discarding whitespace at the beginning and end.

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There's the findstr command. It can search files with regular expressions, much like grep in Linux. It can also search piped input.

@echo off

set param=%1

echo %param%| findstr /r "^[1-9][0-9]*$">nul

if %errorlevel% equ 0 (
    echo Valid number
)

Explanation

The parameter used is set into the param variable. However, there is nothing stopping you using the parameter directly (%1 for the first paramater).

findstr is used to search the piped input with the /r flag for regex.

The pattern:

• ^ means beginning of line.

• [0-9] means a digit. The * means the previous repeated zero or more times. So [0-9][0-9]* means one digit, plus zero or more digits. In other words, at least one digit. The + one or more times does not seem to be supported by findstr. Note that [1-9] is used for the first digit to disallow leading zeroes - see the comments.

• $ means end of line.

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Now, a for loop in batch for x number of times... if x is not a valid number, the loop actually does not run at all - it just gets skipped for the next line. So there is no need to check if the input is a valid number!

@echo off

set param=%1

for /l %%a in (1,1,%param%) do (
    echo %%a
)

The loop is done using for /l, where (x,y,z) means start at x, increment by y until z is reached. And it sets %%a to the current number/iteration.

Note: this actually fails if there is a leading zero, causing the command processor to treat it as an octal number. See dbenham's answer for a better solution.

The following works very well for me. SET /a param=%1+0 always returns 0 if %1 is empty or not numeric. Otherwise, it provides the given number.

SET /a param=%1+0
IF NOT %param%==0 ECHO Valid number

This will detect if the first parameter is a valid natural number (non-negative integer).

@echo off
echo %1|findstr /xr "[1-9][0-9]* 0" >nul && (
  echo %1 is a valid number
) || (
  echo %1 is NOT a valid number
)

If you want to allow quotes around the number, then

@echo off
echo "%~1"|findstr /xr /c:\"[1-9][0-9]*\" /c:\"0\" >nul && (
  echo %~1 is a valid number
) || (
  echo %~1 is NOT a valid number
)

Note - leading zeros are disallowed because batch treats them as octal, so a value like 09 is invalid, and 010 has a value of 8.