Got it, let's break down your requirements and build the exact regex you need. First, let's restate your rules clearly to make sure we're on the same page:

• Your string can only contain the special characters % and & — any other character (like @, $, letters, numbers, or other symbols) should make validation fail
• The string must have 2 or more total instances of % and/or & (so no single %, single &, or empty strings allowed)

The Regex You Need #

^[%&]{2,}$

How This Regex Works #

Let's break down each component so you understand exactly what it's doing:

• ^: Anchors the match to the start of the string — this makes sure there aren't any hidden, invalid characters before your allowed ones
• [%&]: A character class that matches only a % or a & — this automatically excludes every other character by default
• {2,}: A quantifier that requires the preceding character class to appear at least 2 times — this enforces your "2+ special characters" rule
• $: Anchors the match to the end of the string — this ensures there aren't any invalid characters hiding after your allowed ones

Test Cases to Verify #

Valid (Matches Successfully)

• %%
• &&
• %&
• &%
• %%&%&

Invalid (Fails Validation)

• % (only 1 special character)
• & (only 1 special character)
• %@ (contains forbidden @)
• $& (contains forbidden $)
• abc% (includes letters)
• Empty string (no special characters at all)

Why This Is Better Than Your Original Regex #

Your original regex ^((%&)*(?!@).)*$ allowed zero or more %/& and blocked @, but it still let other characters slip through thanks to the . wildcard (which matches any character). This new regex tightens things up to only allow % and &, and enforces the minimum count requirement you need.

内容的提问来源于stack exchange，提问作者Delfin