Regular expression pipe confusion

Question

How come this code returns true?

string to match: ab

pattern: /^a|b$/

but when I put parentheses like this:

pattern: /^(a|b)$/

it will then return false.

PHP, C++, Python, which regex implementation are you asking about? (Looks like PHP PCRE to me.) — BoltClock
– BoltClock, Commented Jun 8, 2011 at 13:22
I believe they have the same implementation regarding the code I've provided so I've put them on the tags :/ — Rei
– Rei, Commented Jun 8, 2011 at 13:24

Daniel Hilgarth · Accepted Answer · 2013-11-27 15:45:57Z

The first pattern without the parenthesis is equivalent to /(^a)|(b$)/.
The reason is, that the pipe operator ("alternation operator") has the lowest precedence of all regex operators: http://www.regular-expressions.info/alternation.html (Third paragraph below the first heading)

Community · Accepted Answer · 2017-05-23 12:34:15Z

/^a|b$/ matches a string which begins with an a OR ends with a b. So it matches afoo, barb, a, b.

/^(a|b)$/ : Matches a string which begins and ends with an a or b. So it matches either an a or b and nothing else.

This happens because alteration | has very low precedence among regex operators.

Related discussion

AProgrammer · Accepted Answer · 2011-06-08 13:23:58Z

5

The first means begin by an a or end with a b.

The second means 1 character, an a or a b.

answered Jun 8, 2011 at 13:23

AProgrammer

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Comments

Cobra_Fast · Accepted Answer · 2011-06-08 13:25:16Z

In ^a|b$ you are matching for an a at the beginning or a b at the end.

In ^(a|b)$ you are matching for an a or a b being the only character (at beginning and end).

Blindy · Accepted Answer · 2011-06-08 13:34:56Z

2

| has lower priority than the anchors, so you're saying either ^a or b$ (which is true) as opposed to the 2nd one which means "a single character string, either a or b" (which is false).

edited Jun 8, 2011 at 13:34

answered Jun 8, 2011 at 13:24

Blindy

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3 Comments

Daniel Hilgarth Over a year ago

it has a lower priority, not a higher one!

Blindy Over a year ago

Er, sure, I meant it's evaluated before :) I've always found this lower/higher thing arbitrary and counter-intuitive...

Wiseguy Over a year ago

But it's not evaluated before -- it's evaluated after. Lowest priority is evaluated last, highest is first. That should be intuitive. I think it's the "before/after" terminology that's causing the confusion, since outermost/biggest actually suggests lower priority.

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Regular expression pipe confusion

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