You only get 12.5 as a match, as there is not /g global flag, but if you would enable the global flag it will give partial matches as there are no anchors ^ and $ in the pattern validating the whole string.

The [+-] is only matched once, which should be repeated to match it multiple times.

Currently the pattern will match 1+2+3 but it will also match 1a1+2b2 as the dot is not escaped and can match any character (use \. to match it literally).

For starting with digits and optional decimal parts and repeating 1 or more times a + or -:

^\d+(?:\.\d+)?(?:[-+]\d+(?:\.\d+)?)+$

Regex demo

If the values can start with optional plus and minus and can also be decimals without leading digits:

^[+-]?\d*\.?\d+(?:[-+][+-]?\d*\.?\d+)+$

• ^ Start of string
• [+-]? Optional + or -
• \d*\.\d+ Match *+ digits with optional . and 1+ digits
• (?: Non capture group
  • [-+] Match a + or -
  • [+-]?\d*\.\d+ Match an optional + or - 0+ digits and optional . and 1+ digits
• )+ Close the noncapture group and repeat 1+ times to match at least a single + or -
• $ End of string

Regex demo

You would try to use this solution for PCRE compatible RegExp engine:

^(?:(-?\d+(?:[\.,]{1}\d)?)[+-]?)*(?1)$

• ^ Start of String
• (?: Non capture group ng1
• (-?\d+(?:[\.,]{1}\d)?) Pattern for digit with or without start "-" and with "." or "," in the middle, matches 1 or 1.1 or 1,1 (Matching group 1)
• [+-]? Pattern for "+" or "-"
• )* Says that group ng1 might to repeat 0 or more times
• (?1) Says that it must be a digit in the end of pattern by reference to the first subpattern
• $ End of string

As JS does not support recursive reference, you may use full version instead:

/^(?:(-?\d+(?:[\.,]{1}\d)?)[+-]?)*(-?\d+(?:[\.,]{1}\d)?)$/gm