How can I find and replace dates written in one consistent text format?

Start by describing the exact shape: two digits, a hyphen, two digits, another hyphen, then four digits. In regex, write that as ([0-9]{2})-([0-9]{2})-([0-9]{4}). Each parenthesized group captures one part: the first two digits, the next two, and the final four. [0-9] specifically means ASCII digits; Python’s \d can also match other Unicode decimal digits, so [0-9] is the clearer choice when you want only 0 through 9. [S1]

This pattern matches the stated shape, not a calendar date. It could match digit groups that do not correspond to a real date. Decide whether you want to find that text anywhere inside a larger string or require a whole-field match; those are different matching tasks.

Use re.sub with a replacement that places the captured year first, then the first and second two-digit groups:

import re

text = "Meeting: 12-31-2025"
pattern = r"([0-9]{2})-([0-9]{2})-([0-9]{4})"
updated = re.sub(pattern, r"\g<3>/\g<1>/\g<2>", text)
print(updated)

The output is Meeting: 2025/12/31. In the replacement, \g<1> refers to the first captured part, \g<2> to the second, and \g<3> to the third. Python documents this capture-reference form for substitution. [S1] If your input may contain other digit strings in the same shape, inspect the output before using it.

For a string containing one occurrence, pass a regular expression with capture groups to replace and use $1, $2, and $3 in the replacement string:

const text = "Meeting: 12-31-2025";
const pattern = /([0-9]{2})-([0-9]{2})-([0-9]{4})/;
const updated = text.replace(pattern, "$3/$1/$2");
console.log(updated);

That produces Meeting: 2025/12/31. JavaScript replacement strings use numbered capture references when replace receives a regular expression. [S2] The example pattern can match a date-shaped substring inside longer text; it does not establish that the parts are a valid date or that a match is intended. Try the change on a copy and review the result.

Treat matching and date validation as separate steps. This regex confirms only the widths and hyphen positions of the text components; it does not prove that they form a real calendar date. [S1][S2] If your task requires valid dates, use an appropriate date-parsing or validation step for your programming environment, and decide how it should handle invalid input before replacing anything. These pattern and replacement examples do not specify those validation rules.

A small review checklist helps prevent a successful text replacement from being mistaken for a correct date conversion: confirm the input convention, test representative examples, and inspect the changed output.