What is a regex to match ONLY an empty string

Question

There are lots of posts about regexs to match a potentially empty string  but I couldn t readily find any which provided a regex which only matched an empty string   I know that   will match the beginning of any line and   will match the end of any line as well as the end of the string  As such       matches far more than the empty string such as   n    foobar n n   etc   I would have thought  though  that   A Z  would match just the empty string  since  A matches the beginning of the string and  Z matches the end of the string  However  my testing shows that   A Z  will also match   n   Why is that

User · Answer

Wow  ya ll are overthinking it  It s as simple as the following  Besides  many of those answers aren t understood by the RE2 dialect used by C and golang

User · Answer

As  Bohemian and  mbomb007 mentioned before  this works AND has the additional advantage of being more readable   console log         s test         true

User · Answer

Try looking here   https   docs python org 2 library re html  I ran into the same problem you had though  I could only build a regex that would match only the empty string and also   n   Try trimming replacing the newline characters in the string with another character first   I was using http   pythex org  and trying weird regexes like these                       n  0    and so on

User · Answer

Another possible answer considering also the case that an empty string might contain several whitespace characters for example spaces tabs line break characters can be the folllowing pattern  pattern   r quot    s    quot  This pattern matches if the string starts and ends with zero or more whitespace characters  It was tested in Python 3

User · Answer

I believe Python is the only widely used language that doesn t support  z in this way  yet   There are Python bindings for Russ Cox   Google s super fast re2 C   library that can be  dropped in  as a replacement for the bundled re    There s an excellent discussion  with workarounds  for this at Perl Compatible Regular Expression  PCRE  in Python  here on SO   python Python 2 7 11  default  Jan 16 2016  01 14 05    GCC 4 2 1 Compatible FreeBSD Clang 3 4 1 on freebsd10 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import re2 as re  gt  gt  gt    gt  gt  gt  re match r  A z        lt re2 Match object at 0x805d97170 gt     tchrist s answer is worth the read

User · Answer

-- regex to accept empty string And it wont match   n  or  foobar n  as you mentioned  You could test this regex on https   www regextester com 1924   If you have your existing regex use or    in your regex to match empty string  For example    A-Za-z0-9 amp

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The answer may be language dependent  but since you don t mention one  here is what I just came up with in js    var a     1      2      3   join   n      console log a match     0   gm                     the     is for readability  it doesn t really matter  a match    you can put whatever the hell you want and this will also work just the same  0   gm    You could also do a match      10     0    gm  to match empty lines OR lines that meet a criteria    This is what I was looking for to end up here       I know that   will match the beginning of any line and   will match the end of any line   This is only true if you have the multiline flag turned on  otherwise it will only match the beginning end of the string   I m assuming you know this and are implying that  but wanted to note it here for learners

User · Answer

I would use a negative lookahead for any character         s S     This can only match if the input is totally empty  because the character class will match any character  including any of the various newline characters

User · Answer

As explained in http   www regular-expressions info anchors html under the section  Strings Ending with a Line Break    Z will generally match before the end of the last newline in strings that end in a newline  If you want to only match the end of the string  you need to use  z  The exception to this rule is Python   In other words  to exclusively match an empty string  you need to use   A z

User · Answer

Based on the most-approved answer  here is yet another way   var result       d D   test string        d D  will match any character

[regex] What is a regex to match ONLY an empty string?

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