/^\d{1,2}(\.\d{1,2})?$/

This regular expression matches a decimal number with one or two digits before the decimal point, and optionally one or two digits after the decimal point.

Here is a breakdown of the different parts of the expression:

• ^ - Matches the start of the string.
• \d{1,2} - Matches one or two digits (0-9).
• (\.\d{1,2})? - Matches an optional group that starts with a decimal point (\.) and is followed by one or two digits. The ? at the end makes the group optional.
• $ - Matches the end of the string.

Examples of valid matches for this regular expression include:

• 0.5
• 12
• 1.23
• 99.99

Examples of invalid matches include:

• 123 (too many digits before the decimal point)
• 0.1234 (too many digits after the decimal point)
• .5 (missing digits before the decimal point)