Dart Regexp replace all but numbers and allow once . or ,
You can use
text.replaceAllMapped(RegExp(r'^([^,.]*[.,])|\D+'), (Match m) => m[1] != null ? m[1].replaceAll(RegExp(r'[^0-9,.]+'), '') : '')The ^([^.,]*[.,])|\D+ regex matches
^([^,.]*[.,])- start of string and then any chars other than,and.and then a.or,captured into Group 1|- or\D+- any one or more non-digit chars.
If the first alternative matches, the , and . should be preserved, so .replaceAll(RegExp(r'[^0-9,.]+'), '') is used to remove all chars other than digits, . and ,.
If \D+ matches, it means the first . or , has already been matched, so the replacement for this alternative is an empty string.