Develop Reference

Regexp adds invisible dot character replacing \b

I want this to be my regex: /^word\b/ (word is dynamic)
When I set it up to be dynamic I have to use this:
var word='spoon';
'spoon .table .chair'.match(new RegExp('^'+word+'\b'));
However, this finds null, while this:
var word='spoon';
'spoon .table .chair'.match(/^spoon\b/);
finds ["spoon"].
The interesting part is when I examine the difference between the regex I worte and the regex RegExp wrote:
console.log(/^spoon\b/,new RegExp('^'+word+'\b'))
It shows this:
/^spoon\b/ /^spoon/
If I then copy the second part of the log output (/^spoon/) into my code editor I see this character:
What is that? How do I do RegExp word-ending-with as I am not always guaranteed to have a space at the end when the string might be a one-word string (spoon or another word)
I'd rather just do this without the invisible thing

You've got to escape the \ in the b in the regex string by adding an extra slash:
var regex = new RegExp('^' + word + '\\b')
This is because the RegExp is expecting to see the two characters \ and b, but the string '\b' is one character, ascii 8, the backspace character (in the same way that '\n' is a single newline character).

In Javascript, \b doesn't mean a \ followed by a b. It means the backspace character (ASCII code 8). To get a \ followed by a b, you need to escape the slash so that Javascript doesn't parse it as a backspace:
'^' + word + '\\b'
The same thing applies if you want to use \d or \s or anything else: You need to escape the \ with another one so that Javascript doesn't think it's a Javascript escape code and the RegExp can parse it as what you expect.

Replace '\' with '-' in a string

I have seen all the questions asked previously but it didn't help me out . I have a string that contains backslash and i want to replace the backslashes with '-'
var s="adbc\sjhf\fkjfh\af";
s = s.replace(/\\/g,'-');
alert(s);
I thought this is the proper way to do it and of course i am wrong because in alert it shows adbcsjhffkjfhaf but i need it to be like adbc-sjhf-fkjfh-af.
What mistake i do here and what is the reason for it and how to achieve this...??
Working JS Fiddle

Your s is initially adbcsjhffkjfhaf. You meant
var s="adbc\\sjhf\\fkjfh\\af";

You need to double-up the backslashes in your input string:
var s="adbc\\sjhf\\fkjfh\\af";
Prefixing a character with '\' in a string literal gives special meaning to that character (eg '\t' means a tab character). If you want to actually include a '\' in your string you must escape it with a second backslash: '\\'

Javascript is ignoring the \ in \s \f \a in your string. Do a console.log(s) after assigning, you will understand.
You need to escape \ with \\. Like: "adbc\\sjhf\\fkjfh\\af"

The string doesn't contain a backslash, it contains the \a, \s and \f (escape sequence for Form Feed).
if you change your string to adbc\\sjhf\\fkjfh\\af
var s="adbc\\sjhf\\fkjfh\\af";
s = s.replace(/\\/g,'-');
alert(s);
you will be able to replace it with -

unterminated string literal in the variable

var MM = '\' + obj[0]['MM '] + '/';
I get two errors while using this code...
missing; before statement and
unterminated string literal

The character \ is "special" because it's used to allow the use of all printable characters in strings. In your case '\' is not a string composed by the only character \, but the beginning of a string starting with the single quote character '.
For exampe if you want the string Hello Andrea "6502" Griffini you can use single quotes
string1 = 'Hello Andrea "6502" Griffini';
and if you want single quotes in the string you can do the opposite
string2 = "Hello Andrea '6502' Griffini";
But what if you want both kind of quotes in the same string? This is where the escape \ character comes handy:
string3 = "'llo Andrea \"6502\" Griffini";
Basically \ before a quote or double quote in a string tells javascript that the following character is just a regular character, with no special meaning attached to it.
Note that the very same character is also used in regular expressions... for example if you want to look for an open bracket [ you must prefix it with a backslash because [ in a regular expression has a special meaning.
The escape is also used to do the opposite... in a string if you put a backslash in front of a normal character you are telling javascript that that character is indeed special... for example
alert("This is\na test");
In the above line the \n sequence means a newline code, so the message displayed will be on two lines ("This is" and "a test").
You may now wonder... what if I need a backslash character in my string? Just double it in that case. In your code for example just use '\\'.
Here is a table for the possible meanings of backslash in strings
\" just a regular double-quote character, it doesn't end the string
\' just a regular single-quote character, it doesn't end the string
\b a backspace character (ASCII code 0x08)
\t a tab character (ASCII code 0x09)
\n a newline character (ASCII code 0x0A)
\v a vertical tab character (ASCII code 0x0B)
\f a form feed character (ASCII code 0x0C)
\r a carriage return character (ASCII code 0x0D)
\033 the character with ASCII code 033 octal = 27 ("ESC" in this case)
\x41 the character with ASCII code 0x41 = 65 ("A" in this case)
\u05D0 the unicode character 0x05D0 (Aleph from the Hebrew charset)
\\ just regular backslash character, not an escape prefix

\ is an escape character. You'll have to double it to literally mean a backslash character, otherwise it'll augment the following character (In this case the next single quote)

You need to properly escape the backslash:
var lastMenstrualPeriod = '\\' + obj[0]['LastMenstrualPeriod'] + '/';
Being escape character, the JS "compiler" is expecting another character to follow, for example \n is newline constant, \t is tab etc.. so \\ is one single backslash in a string.
It is also mentioned in Douglas Crockford book.

You are forgetting to escape '\'
Do this:
var lastMenstrualPeriod = '\\' + obj[0]['LastMenstrualPeriod'] + '/';

javascript: some problem with compiler interpreting // as comments in regex

I've got this regular expression for validating phone numbers
^(\+?|(\(\+?[0-9]{1,3}\))|)([ 0-9.//-]|\([ 0-9.//-]+\))+((x|X|((e|E)(x|X)(t|T)))([ 0-9.//-]|\([ 0-9.//-]+\)))?$
I dugged it out from my C#/vb library and now i want to translate it into javascript. But it has syntax error (i suspect it is something due to the // characters). my attempt:
$IsPhone = function (input) {
var regex = new window.RegExp("^$|^(\+?|(\(\+?[0-9]{1,3}\))|)([ 0-9.//-]|\([ 0-9.//-]+\))+((x|X|((e|E)(x|X)(t|T)))([ 0-9.//-]|\([ 0-9.//-]+\)))?$", "");
return regex.test(input.trim());
};
alert($IsPhone("asd"));

Your problem has nothing to do with comments. You're just mixing up the two different ways of creating RegExp objects.
When you create a RegExp object in JavaScript code, you either write it as a string literal which you pass to a RegExp constructor, or as a regex literal. Because string literals support backslash-escape sequences like \n and \", any actual backslash in the string has to be escaped, too. So, whenever you need to escape a regex metacharacter like ( or +, you have to use two backslashes, like so:
var r0 = "^$|^(\\+?|(\\(\\+?[0-9]{1,3}\\))|)([ 0-9./-]|\\([ 0-9./-]+\\))+((x|X|((e|E)(x|X)(t|T)))([ 0-9./-]|\\([ 0-9./-]+\\)))?$";
var regex0 = new RegExp(r0, "");
The forward-slash has no special meaning, either to regexes or strings. The only reason you ever have to escape forward-slashes is because they're used as the delimiter for regex literals. You use backslashes to escape the forward-slashes just like you do with regex metacharacters like \( or \+, or the backslash itself: \\. Here's the regex-literal version of your regex:
var regex1 = /^$|^(\+?|(\(\+?[0-9]{1,3}\))|)([ 0-9.\/-]|\([ 0-9.\/-]+\))+((x|X|((e|E)(x|X)(t|T)))([ 0-9.\/-]|\([ 0-9.\/-]+\)))?$/;

from Errors translating regex from .NET to javascript
The backslash character in JavaScript
strings is an escape character, so the
backslashes you have in your string
are escaping the next character for
the string, not for the regular
expression. So right near the
beginning, in your "^(+?, the
backslash there just escapes the + for
the string (which it doesn't need),
and what the regexp sees is just a raw
+ with nothing to repeat. Hence the error.

Finding Plus Sign in Regular Expression

var string = 'abcd+1';
var pattern = 'd+1'
var reg = new RegExp(pattern,'');
alert(string.search(reg));
I found out last night that if you try and find a plus sign in a string of text with a Javascript regular expression, it fails. It will not find that pattern, even though it exists in that string. This has to be because of a special character. What's the best way to find a plus sign in a piece of text? Also, what other characters will this fail on?

Plus is a special character in regular expressions, so to express the character as data you must escape it by prefixing it with \.
var reg = /d\+1/;

\-\.\/\[\]\\ **always** need escaping
\*\+\?\)\{\}\| need escaping when **not** in a character class- [a-z*+{}()?]
But if you are unsure, it does no harm to include the escape before a non-word character you are trying to match.
A digit or letter is a word character, escaping a digit refers to a previous match, escaping a letter can match an unprintable character, like a newline (\n), tab (\t) or word boundary (\b), or a a set of characters, like any word-character (\w), any non-word character (\W).
Don't escape a letter or digit unless you mean it.

Just a note,
\ should be \\ in RegExp pattern string, RegExp("d\+1") will not work and Regexp(/d\+1/) will get error.
var string = 'abcd+1';
var pattern = 'd\\+1'
var reg = new RegExp(pattern,'');
alert(string.search(reg));
//3

You should use the escape character \ in front of the + in your pattern. eg. \+

You probably need to escape the plus sign:
var pattern = /d\+1/
The plus sign is used in regular expressions to indicate 1 or more characters in a row.

It should be var pattern = '/d\\+1/'.
The string will escape '\\' as '\' ('\\+' --> '\+') so the regex object init with /d\+1/

if you want to use + (plus sign) or $ (sigil /dollar sign), then use \ (backslash) as a prefix. Like that:
\$ or \+