What is the difference between square brackets and parentheses in a regex

Question

Here is a regular expression I created to use in JavaScript   var reg num      7 8 9  d 9      Here is another one suggested by my team member   var reg num      7 8 9   d  9      The rule is to validate a phone number    It should be of only ten numbers    The first number is supposed to be any of 7  8 or 9

User · Answer

The first 2 examples act very differently if you are REPLACING them by something. If you match on this:

str = str.replace(/^(7|8|9)/ig,'');

you would replace 7 or 8 or 9 by the empty string.

If you match on this

str = str.replace(/^[7|8|9]/ig,'');

you will replace 7 or 8 or 9 OR THE VERTICAL BAR!!!! by the empty string.

I just found this out the hard way.

User · Answer

These regexes are equivalent  for matching purposes        7 8 9  d 9       789  d 9       7-9  d 9      The explanation     a b c  is a regex  OR  and means  a or b or c   although the presence of brackets   necessary for the OR  also captures the digit  To be strictly equivalent  you would code    7 8 9  to make it a non capturing group   abc  is a  character class  that means  any character from a b or c   a character class may use ranges  e g   a-d     abcd     The reason these regexes are similar is that a character class is a shorthand for an  or   but only for single characters   In an alternation  you can also do something like  abc def  which does not translate to a character class

User · Answer

Your team s advice is almost right  except for the mistake that was made  Once you find out why  you will never forget it  Take a look at this mistake        7 8 9  d 9      What this does      and   denotes anchored matches  which asserts that the subpattern in between these anchors are the entire match  The string will only match if the subpattern matches the entirety of it  not just a section     denotes a capturing group  7 8 9 denotes matching either of 7  8  or 9  It does this with alternations  which is what the pipe operator   does     alternating between alternations  This backtracks between alternations  If the first alternation is not matched  the engine has to return before the pointer location moved during the match of the alternation  to continue matching the next alternation  Whereas the character class can advance sequentially  See this match on a regex engine with optimizations disabled      Pattern   r f at Match string  carat     Pattern   rf at Match string  carat       d 9  matches nine digits   d is a shorthanded metacharacter  which matches any digits         7 8 9   d  9      Look at what it does      and   denotes anchored matches as well   7 8 9  is a character class  Any characters from the list 7     8     or 9 can be matched  thus the   was added in incorrectly  This matches without backtracking    d  is a character class that inhabits the metacharacter  d  The combination of the use of a character class and a single metacharacter is a bad idea  by the way  since the layer of abstraction can slow down the match  but this is only an implementation detail and only applies to a few of regex implementations  JavaScript is not one  but it does make the subpattern slightly longer   9  indicates the previous single construct is repeated nine times in total    The optimal regex is    789  d 9     because    7 8 9  d 9    captures unnecessarily which imposes a performance decrease on most regex implementations  javascript happens to be one  considering the question uses keyword var in code  this probably is JavaScript   The use of php which runs on PCRE for preg matching will optimize away the lack of backtracking  however we re not in PHP either  so using classes    instead of alternations   gives performance bonus as the match does not backtrack  and therefore both matches and fails faster than using your previous regular expression

[regex] What is the difference between square brackets and parentheses in a regex?

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