#Hashtag_or_not

Question

In this code golf challenge, you will verify hashtags!

#What_your_code_should_do

Input is a string. Output a truthy value if it is a valid hashtag, and a falsy value otherwise.

We define a string as a valid Hashtag if ...

• It starts with a hash (#).
• It doesn't have a number right after the hashtag (e.g. #2016USElection isn't a valid hashtag).
• It doesn't have any "special characters" (i.e. any character that isn't an alphabet, underscore (_) or a number).

You can assume that the input only contains ASCII characters. (It would be unfair if we did Unicode too.)

#Rules

Basic code-golf rules apply.

#Examples

Truthy:

#
#e
#_ABC 
#thisisanunexpectedlylongstringxoxoxoxo
#USElection2016

Falsy:

Hello, World!
#12thBday
#not-valid
#alsoNotValid!
#!not_a_hash

Answer 1

Retina, 12 bytes

^#(?!\d)\w*$

Prints 1 for hashtags and 0 otherwise.

Try it online! (The first line enables a linefeed-separated test suite.)

Not much to explain here, this is quite a literal implementation of the definition: ^ and $ are just anchors ensuring that the match covers the entire string, # checks that the string starts with a #, (?!\d) ensures that the next character isn't a digit (without advancing the regex engine's position), \w* checks that we can reach the end of the string with zero or more letters, digits or underscores.

By default, Retina counts the number of matches of the given regex, which is why this gives 1 for valid hash tags and 0 otherwise.

Answer 2

Perl, 22 bytes

21 bytes code +1 for -p

$_=/^#([a-z_]\w*)?$/i

Prints 1 if it's a valid hashtag, empty string otherwise.

Usage

perl -pe '$_=/^#([a-z_]\w*)?$/i' <<< '#'
1
perl -pe '$_=/^#([a-z_]\w*)?$/i' <<< '#_test'
1
perl -pe '$_=/^#([a-z_]\w*)?$/i' <<< '#1test'

Saved 2 bytes thanks for Martin Ender (and another 4 using his lookaround method)

---

Perl, 18 bytes

17 bytes code +1 for -p

Using Martin's lookaround this can be much shorter!

$_=/^#(?!\d)\w*$/

Answer 3

JavaScript (ES6), 25 bytes

s=>/^#(?!\d)\w*$/.test(s)

F = s => /^#(?!\d)\w*$/.test(s)
input.oninput = () => result.innerHTML = input.value ? F(input.value) ? '\ud83d\udc8e' : '\ud83d\udca9' : '\ud83d\udcad';

#input, #result {
  vertical-align: middle;
  display: inline-block;
}
#input {
  line-height: 2em;
}
#result {
    font-size: 2em;
}

<input id="input" type="text"/> <span id="result">&#x1f4ad</span>

Answer 4

C, 80 bytes

Function f() takes the string as an argument and modifies int *b to either 1 or 0 to indicate truthy/falsy.

f(char*p,int*b){for(*b=(*p==35)&&!isdigit(p[1]);*p;++p)*b&=isalnum(*p)||*p==95;}

If the string always has at least one character (i.e. never an empty string), a byte can be shaved off for 79 bytes:

f(char*p,int*b){for(*b=(*p==35)&&!isdigit(p[1]);*++p;)*b&=isalnum(*p)||*p==95;}

Answer 5

Python 3, 41 bytes

import re
re.compile('#(?!\d)\w*$').match

Answer 6

Brachylog, 55 bytes

"#"|h"#",?b@lL'(eE,@A:"1234567890":"_"c'eE),@A:"_"ce~hL

This uses no regex.

Explanation

Main predicate, Input (?) is a string

  "#"                           ? = "#"
|                             Or
  h"#",                         First character of ? is "#"
  ?b@lL                         L is the rest of the chars of ? lowercased
  '(                            It is not possible for any char of L that...
    eE,                           Call this char E
    @A:"1234567890":"_"c          Concatenate the lowercase alphabet with the digits and "_"
    'eE                           E is not a member of that concatenated string
   ),                           
   @A:"_"c                      Concatenate the lowercase alphabet with "_"
   e~hL                         One char of that concatenated string is the first char of L

Answer 7

Python 3, 103 93 bytes

all((c=='_'or c.isalpha()*i>0)^(i<1and'#'==c)^(c.isdigit()*i>1)for i,c in enumerate(input()))

The # being True killed me here, I had to enumerate the string to avoid an index error on the single character input.

Answer 8

PowerShell v2+, 25 bytes

$args-match'^#(?!\d)\w*$'

Using Martin's regex, just wrapped up in PowerShell's -match operator coupled with the input $args. For truthy/falsey values, this will return the string itself on a match (a truthy value) or nothing on a non-match (a falsey value). This is because when a comparison operator is applied against an array, it returns anything that satisfies that operator.

A couple examples (wrapped in a [bool] cast to make the output more clear):

PS C:\Tools\Scripts\golfing> [bool](.\hashtag-or-not.ps1 '#2016Election')
False

PS C:\Tools\Scripts\golfing> [bool](.\hashtag-or-not.ps1 'Hello, World!')
False

PS C:\Tools\Scripts\golfing> [bool](.\hashtag-or-not.ps1 '#')
True

PS C:\Tools\Scripts\golfing> [bool](.\hashtag-or-not.ps1 '')
False

PS C:\Tools\Scripts\golfing> [bool](.\hashtag-or-not.ps1 '#USElection2016')
True

Answer 9

Mathematica, 52 46 43 bytes

^{Saved 6 9 bytes due to @MartinEnder.}

StringMatchQ@RegularExpression@"#(?!\d)\w*"

Function. Takes a string as input, and returns True or False as output. Pretty simple, just matches against the regex /#(?!\d)\w*/.

Answer 10

Python3 - 156 128 bytes

lambda n:n=="#"or(n[0]=="#")*all(any([47<ord(c)<58,64<ord(c)<91,ord(c)==95,96<ord(c)<123])for c in n[1:]+"0")*~(47<ord(n[1])<58)

A solution that doesn't use regex. 0 is falsey and every other value is truthy.

Thanks to @LeakyNun for saving bytes!

Answer 11

Dyalog APL, 22 20 bytes

Without RegEx:

{0≤⎕NC 1↓⍵,⎕A}∧'#'=⊃

-2 thanks to ngn

Answer 12

Octave, 37 56 54 43 bytes

Thanks to @LuisMendo for removing 8 bytes!

@(s)s(1)==35&(isvarname(s(2:end))|nnz(s)<2)

Not very golfy, but very built-inny.
Edit: The original code accepted strings with no leading '#'. I guess I should have stuck with regex.

Test suite on ideone.

Answer 13

Python 2, 79 bytes

lambda x:x=='#'or(1>x[1].isdigit())&x[1:].replace('_','').isalnum()&('#'==x[0])

First golfing attempt. Ungolfed version:

def f(x):
    if x == '#':
        return True
    else:
        return x[0]=='#' and x[1:].replace('_','').isalnum() and not x[1].isdigit()

Answer 14

Google Sheets, 30 bytes

An anonymous worksheet function that takes input from the cell A1 checks it against the RE2 expression and outputs the result to the calling cell.

=RegexMatch(A1,"^#([a-z_]\w*)?

Answer 15

Lua, 59 55 54 bytes

Code

s=arg[1]print(load(s:sub(2).."=0")and s:sub(1,1)=="#")

How it works:

1. Check if the rest of the characters can be a valud Lua identifier (identifiers in Lua follow the same rules as hashtags.)
2. Check if the first character is a #.

Takes input from the command line. Prints true if the string is a valid hashtag, otherwise, it prints nil.

Answer 16

Excel VBA, 54 bytes

Anonymous VBE immediate window function that takes input from cell [A1], checks if the value of the cell matches the Like pattern, and outputs as Boolean to the VBE immediate window

?Not[Left(A1,2)]Like"[#]#"And[A1]Like"[#][_a-zA-z0-9]*

Answer 17

Haskell, 79 bytes

import Data.Char
f(a:r)=a=='#'&&r==""||r!!0>'@'&&all(\c->c=='_'||isAlphaNum c)r

Try it online!

Answer 18

05AB1E, 18 bytes

Code:

¬'#Qs¦A«¬d_sDžjKQP

Uses the CP-1252 encoding. Try it online!.

Answer 19

Standard ML, 121 118 107 bytes

(fn#"#"::a=>(fn x::r=>x> #"@"andalso List.all(fn#"_"=>1=1|c=>Char.isAlphaNum c)a|e=>1=1)a|e=>1=0)o explode;

Try it online! Functional solution without using regex. Declares an anonymous function which is bond to the implicit result identifier it.

> val it = fn : string -> bool    
- it "#valid_hash";
> val it = true : bool

Answer 20

Pyke, 19 bytes

\#.^It,!It\_D-PRkq|

Try it here!

Quick fix for tonight

Answer 21

Python 3, 97 Bytes 70 Bytes 56 Bytes

lambda x:s=x[2:];b=x[1];all(x!="#",[x[0]=="#",any[b.isalpha(),"_"in b],any[s.isalnum(),"_"in s]])

(Code changed) Human readable

x=input()
if x[0]=="#" and x[1].isalpha() and str(x)[2:].isalnum():
    print(True)
else:
    print(False)

Answer 22

GNU grep, 15 + 2 = 17 bytes

grep -Ei '^#([a-z_]\w*)?$'

Test:

$ echo '#
#e
#_ABC
#thisisanunexpectedlylongstringxoxoxoxo
#USElection2016
Hello, World!
#12thBday
#not-valid
#alsoNotValid!' | grep -Ei '^#([a-z_][a-z0-9_]*)?$'

Output:

#
#e
#_ABC
#thisisanunexpectedlylongstringxoxoxoxo
#USElection2016

Answer 23

Sed 19 + 2 = 21 bytes

/^#([a-z_]\w*)?$/Ip

This filters out all non-hashtags and outputs valid hashtags.

Run as sed -rn "/^#$|^#[a-z]\w*$/Ip". Quit with Ctrl + D (send EOF).

Answer 24

Ruby, 16 + 3 1 (n flag) = 19 17 bytes

Uses 0 as truthy and nil as falsy.

p~/^#(?!\d)\w*$/

Run it as ruby -ne 'p~/^#(?!\d)\w*$/'. Thanks to @manatwork for fixing the bash error when running the program.

Answer 25

Clojure, 130 135 132 bytes

• +5 bytes to deal with an NPE that happened when the string consisted of only a hashtag.

• -2 bytes by using Character/isLetterOrDigit.

(fn[s](let[[h & r]s n(map int r)](and(= h\#)(not(<= 48(or(first n)0)57))(every? #(or(Character/isLetterOrDigit^long %)(= 95 %))n))))

Ungolfed:

(defn hashtag? [s]
  (let [[h & r] s
        codes (map int r)]
    (and (= h \#)
         (not (<= 48 (or (first codes) 0) 57))
         (every?
           #(or (Character/isLetterOrDigit ^long %)
                (= 95 %))
           codes))))

Answer 26

Java 8, 57 54 28 bytes

s->s.matches("#(?!\\d)\\w*")

Port of Martin Ender's Retina answer to save a few bytes and match added test cases.
Not that String#matches always matches the entire String, so no need for ^...$.

Try it here.

Answer 27

C#, 92 bytes

s=>s[0]=='#'&s.Length>1&&(s[1]<48|s[1]>57)&s.Skip(1).All(x=>char.IsLetterOrDigit(x)|x=='_');

C# lambda (Predicate) where input is a string and output is a bool.

Try it online!

Answer 28

Lua, 39 bytes

print(arg[1]:match("^#[%a_][%a_%d]*$"))

Straightforward copypasta of match description. Outputs falsy nil if not hashtag, outputs truthly hashtag back otherwise.

Can be shortened for one more byte by using find if outputing list of two values (which is truthly) doesn't break rules.