💥 Regular Expressions

One statement, no questions,
Put your fist up for your regular expressions.
—Dual Core, Regular Expressions

Regular expressions are powerful.

Regular expressions are fun.

Right up until they are not. Today was one such day.

At some point, anyone who’s had to work with regular expressions has needed to match a specific range of characters. Let’s say—for no particular reason—alpha characters. A to Z. Or a to z. The basics, none of those fancy Unicode ones, thankyouverymuch.

$ grep --perl-regexp 'i' <<< "testing"
testing

$ grep --perl-regexp 'a' <<< "testing"

To match all characters, you’d need to include them all somehow. You could, of course, go all-in:

$ grep --perl-regexp '[abcdefghijklmnopqrstuvwxyz]' <<< "testing"
testing

…which would work, but it’s not very concise, is it? You could, of course, use a character class and match any word character:

$ grep --perl-regexp '\w' <<< "testing"
testing

…which would, again, work but you could accidentally match a non-ASCII and you wouldn’t want that, would you?

Of course, you could also use one of the convenient character ranges:

$ grep --perl-regexp '[a-z]' <<< "testing"
testing

Wonderful! But that won’t match upper-case characters, will it?

grep --perl-regexp '^[a-z]+$' <<< "Testing"

Hmmmm…no. Using the upper-case variant would leave us in the same pickle, no?

grep --perl-regexp '^[A-Z]+$' <<< "Testing"

Curses! But, of course, there’s another option: a combination of the two! How convenient!

grep --perl-regexp '^[A-z]+$' <<< "Testing"
Testing

It worked perfectly! Matching only alpha. characters and both casing variants! All is well, right?

Right?!

No

grep --perl-regexp '^[A-z]+$' <<< "Test[\]^_`ing"
Test[\]^_`ing

Errrr…what? A-z. That’s fairly clear, right?

No. No, it is not. Because A-z is not, in fact, shorthand for “the range of characters from a to a, upper- and lower-case.” It is, in fact, the shorthand for “the range of ASCII characters from A to z”.

$ python3 -c 'print("".join(chr(c) for c in range(ord("A"), ord("z") + 1)))'
ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxy

Oh dear. Oh dear, oh dear.

Quite what was going on at the American Standards Association between Z and a, I’d hate to guess. Presumably their cat walked across the keyboard while they were celebrating the addition of the capital letters.

But yes, there’s a whole bunch of punctuation (^ and ` are technically symbols, not punctuation; fun fact) smack in the middle of those character ranges which will get matched should you ever opt to use A-z in a regular expression.

Which no one would, right?

Right?!