Question

The following line is in the jQuery source code:

// Used for matching numbers
core_pnum = /[\-+]?(?:\d*\.|)\d+(?:[eE][\-+]?\d+|)/.source,

I am confused by the last vertical bar in the two non-capturing groups,

(?:\d*\.|) 

and

(?:[eE][\-+]?\d+|)

The regular expression /(a|b)/ matches a or b, so I wondered what /(a|b|)/ matches, and it seems to “match everything”, in other words

reg1 = /(a|b)/;
reg1.test('c'); // false
reg2 = /(a|b|)/;
reg2.test('c'); // true

What is going on?

Answer 1

I’ll try to break this down into chunks:

• [\-+]?: This matches a plus sign, a minus sign or neither.
• (?:\d*\.|): This groups (the ?: makes it a non-capturing group) any number of digits followed by a dot or nothing at all.
• \d+: This matches one or more consecutive digits.
• (?:[eE][\-+]?\d+|): This groups a lowercase or uppercase “e”, possibly followed by a plus or a minus and all of that followed by digits. Or, nothing.

(a|) looks for a first. If a doesn’t exist, it matches nothing. It’s a confusing way of writing (a)?.

This regex is a little confusing. I’d re-write it like this:

/[+-]?\d*\.?\d+(?:e[+-]?\d+)?/i