It may be useful if you need to find what appear to be unescaped ampersands in a string.

“This & that” would match
“This & that” would not match

It doesn’t take into account the validity of the entity reference, and doesn’t account for numeric character entities in hexadecimal form.

I agree with your disagreement with me on item 4. :) I jumped the gun on the number of groups. Good job on the explanations/corrections, too.

As for the “errors”:

1. US phone number — doesn’t account for a preceding 1, if the area code is in parenthesis, if the digit groups are separated by a dot or space instead of a dash, or the fact that cell phones have Q and Z on them. It also doesn’t make sure the group is isolated, and not part of something like 1234888-234-123456123.

 \b((1[-. ])?\(?[a-z0-9]{3}\)?[-. ][a-z0-9]{3}[-.][a-z0-9]{4})\b

2. HTML character and entity references — should be a positive look-ahead, not a negative one, and instead of a word character it should be a-z since entity references don’t have ‘_’

&(?=([a-z]+|\#\d+);)

3. numbers in exponential notation (with ‘1.2354 e10′ style exponent) — I believe that exponential notation only has one digit before the decimal, so the \d* should be dropped and a negative lookbehind added to ensure a single digit. Also, there could be a space between the digits and the exponent.

\b(?<!\d)(-?[0-9])(\.\d+)?\s?([eE][-+ ]?\d+)\b

4. MAC address — doesn’t allow for digit groups to be delineated by hyphens. Because : counts as a non-word character it’s not as easy as putting a word boundary on either side.

(?<![-0-9a-f:])([\da-f]{2}[-:]){5}([\da-f]{2})(?![-0-9a-f:])

There’s also a type of MAC address format like 0123.4567.89ab, so you could more precisely do this:

(?<![-0-9a-f:])(([\da-f]{2}[-:]){5}([\da-f]{2})|([\da-f]{4}\.){2}([\da-f]{4}))(?!\.?[-0-9a-f:])

5. XML style markup tags — shouldn’t have an asterisk for the contents, since that could also match an empty <>. And having the non-greedy * followed by a >, and a match for any character but > were accomplishing the same thing so I dropped one.

(<.+?>)

Note: I’m assuming these will be processed as case-insensitive, or else there’s a whole new set of problems… There are a million different ways to do regex, so these are just my suggestions — I’m sure there are better ways.

This was fun!

2. Not a clue about this one! An optional ampersand, followed by an exclamation mark, followed by either one or more words (alphanumerics) or a hash followed by one or more digits, terminated with a semi-colon.

3. I think this is a number expressed in scientific notation, e.g. 1.3e10 - However, I don’t think the \d is necessary in “[1-9]\d*”

4. This looks like a MAC address. Six hex numbers separated by colons. However, I can’t see anything wrong with it so I’m probably wrong!

5. This looks like it would match an SGML tag of some sort, although it’s not very specific. It will match anything starting with a ” followed by a closing ‘>’.