CODEWITHSUNDEEP
javascriptjqueryregex
jdg
jdg

Reputation: 2298

Clarifcation on Regular Expression in jQuery Source

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?

Upvotes: 2

Views: 590

Answers (3)

Blender
Blender

Reputation: 298136

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

Upvotes: 1

inhan
inhan

Reputation: 7470

[\-+]?                # a plus or a minus sign (optional)
(?:                   # a non-capturing group
    \d*\.             # 0 or more decimals and a dot [cond-A1]
    |                 # or
                      # nothing [cond-A2]
)
\d+                   # 1 or more decimals [decimal]
(?:                   # a non-capturing group
    [eE][\-+]?\d+     # (e or E) + optional(minus or plus) + 1 or more decimals [cond-B1]
    |                 # or
                      # nothing  [cond-B2]
)

Here are some examples (I'm skipping the first optional plus/minus signs).

  • .3 would be [cond-A1][decimal][cond-B2]
  • 1.3 would also be [cond-A1][decimal][cond-B2]
  • 3 would be [cond-A2][decimal][cond-B2]
  • 9.0122222900391E-5 would be [cond-A1][decimal][cond-B1]
  • 6.0221418E23-5 would also be [cond-A1][decimal][cond-B1]

Upvotes: 1

burning_LEGION
burning_LEGION

Reputation: 13450

(a|b|) it is mean a or b or "nothing", 'c' contains "nothing"

Upvotes: 1

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