To test a string for numeric characters

SELECT LENGTH(TRIM(TRANSLATE(string1, ' +-.0123456789', ' '))) FROM DUAL;

OUTPUT

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LENGTH(TRIM(TRANSLATE('123b', ' +-.0123456789',' ')));	would return 1
LENGTH(TRIM(TRANSLATE('a123b', ' +-.0123456789',' ')));	would return 2
LENGTH(TRIM(TRANSLATE('1256.54', ' +-.0123456789',' ')));	would return null
LENGTH(TRIM(TRANSLATE ('-56', ' +-.0123456789',' ')));	would return null