CODEWITHSUNDEEP
regex
KaramJaber
KaramJaber

Reputation: 913

what do the slash mean in the regex expressions?

What is the regexp "[a-z0-9]\+" suppose to mean? Or specifically what is the "\" suppose to mean here? Is it right the no string in the world that matches that regexp?

Upvotes: 3

Views: 476

Answers (5)

gkalpak
gkalpak

Reputation: 48211

The slash ("\") here is used to escape the +. + has a special meaning in regex: it means repeat the precceding item one or more times.

  • [a-z0-9]\+ means: Match one alphanumeric character and a plus sign ("+")
  • [a-z0-9]+ means: Match one or more alphanumeric characters

You can find more info on regex Quantifiers here.
You might also want to take a look at this Regular Expression Basic Syntax Reference.

Upvotes: 0

Kent
Kent

Reputation: 195249

it is hard to say. it depends on which RE (BRE? ERE? or PCRE) do you use.

if you use BRE, the \ gives + special meaning.

if you use ERE/PCRE, \ takes special meaning of + away.

an example would be clear (with grep, default using BRE):

kent$  echo "aaaaaa+"|grep -o 'a+'
a+

kent$  echo "aaaaaa+"|grep -o 'a\+'
aaaaaa

kent$  echo "aaaaaa+"|grep -oE 'a\+'
a+

kent$  echo "aaaaaa+"|grep -oE 'a+' 
aaaaaa

Upvotes: 5

Deepu
Deepu

Reputation: 7610

A backslash (\)escapes special characters to suppress their special meaning. So here the special meaning of + is removed. It is considered as just a symbol. The usual meaning of + is to repeat the preceding block 1 or more times.

So in this case [a-z0-9]\+ means a single lower case letter or a decimal digit followed by a +. a+, b+, 0+, 1+ etc. are all valid expressions of this regular expression.

Upvotes: 0

p.s.w.g
p.s.w.g

Reputation: 149078

The \ in [a-z0-9]\+ means the literal plus character. It's needed because without the slash, the + would be a one-or-more quantifier.

Compare:

  • [a-z0-9]\+ matches a Latin lower-case letter or number followed by a plus, e.g. y+
  • [a-z0-9]+ matches one or more Latin lower-case letter or numbers, e.g. xy4

Upvotes: 2

David M
David M

Reputation: 72930

It makes the "+" a literal + symbol. This would be matched by any character matching the first range followed by a + sign, such as c+ or 5+.

Without the slash, the + would mean "one or more occurences", so strings like abc, t63.

Upvotes: 0

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