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Is there a way to achieve the equivalent of a negative lookbehind in JavaScript regular expressions? I need to match a string that does not start with a specific set of characters.
It seems I am unable to find a regex that does this without failing if the matched part is found at the beginning of the string. Negative lookbehinds seem to be the only answer, but JavaScript doesn't has one.
This is the regex that I would like to work, but it doesn't:
(?<!([abcdefg]))m
So it would match the 'm' in 'jim' or 'm', but not 'jam'
Since 2018, Lookbehind Assertions are part of the ECMAScript language specification.
// positive lookbehind
(?<=...)
// negative lookbehind
(?<!...)
Answer pre-2018
As Javascript supports negative lookahead, one way to do it is:
reverse the input string
match with a reversed regex
reverse and reformat the matches
const reverse = s => s.split('').reverse().join('');
const test = (stringToTests, reversedRegexp) => stringToTests
.map(reverse)
.forEach((s,i) => {
const match = reversedRegexp.test(s);
console.log(stringToTests[i], match, 'token:', match ? reverse(reversedRegexp.exec(s)[0]) : 'Ø');
});
Example 1:
Following #andrew-ensley's question:
test(['jim', 'm', 'jam'], /m(?!([abcdefg]))/)
Outputs:
jim true token: m
m true token: m
jam false token: Ø
Example 2:
Following #neaumusic comment (match max-height but not line-height, the token being height):
test(['max-height', 'line-height'], /thgieh(?!(-enil))/)
Outputs:
max-height true token: height
line-height false token: Ø
Lookbehind Assertions got accepted into the ECMAScript specification in 2018.
Positive lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<=\$)\d+\.\d*/) // Matches "9.99"
);
Negative lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<!\$)\d+\.\d*/) // Matches "8.47"
);
Platform support:
✔️ V8
✔️ Google Chrome 62.0
✔️ Microsoft Edge 79.0
✔️ Node.js 6.0 behind a flag and 9.0 without a flag
✔️ Deno (all versions)
✔️ SpiderMonkey
✔️ Mozilla Firefox 78.0
🛠️ JavaScriptCore: support in beta as of 2023-02-16: the feature has been merged
✔️ Apple Safari 16.4 (in beta as of 2023-02-16)
✔️ iOS 16.4 beta WebView (all browsers on iOS + iPadOS)
✔️ Bun 0.2.2
❌ Chakra: Microsoft was working on it but Chakra is now abandoned in favor of V8
❌ Internet Explorer
❌ Edge versions prior to 79 (the ones based on EdgeHTML+Chakra)
Let's suppose you want to find all int not preceded by unsigned :
With support for negative look-behind:
(?<!unsigned )int
Without support for negative look-behind:
((?!unsigned ).{9}|^.{0,8})int
Basically idea is to grab n preceding characters and exclude match with negative look-ahead, but also match the cases where there's no preceeding n characters. (where n is length of look-behind).
So the regex in question:
(?<!([abcdefg]))m
would translate to:
((?!([abcdefg])).|^)m
You might need to play with capturing groups to find exact spot of the string that interests you or you want to replace specific part with something else.
Mijoja's strategy works for your specific case but not in general:
js>newString = "Fall ball bill balll llama".replace(/(ba)?ll/g,
function($0,$1){ return $1?$0:"[match]";});
Fa[match] ball bi[match] balll [match]ama
Here's an example where the goal is to match a double-l but not if it is preceded by "ba". Note the word "balll" -- true lookbehind should have suppressed the first 2 l's but matched the 2nd pair. But by matching the first 2 l's and then ignoring that match as a false positive, the regexp engine proceeds from the end of that match, and ignores any characters within the false positive.
Use
newString = string.replace(/([abcdefg])?m/, function($0,$1){ return $1?$0:'m';});
You could define a non-capturing group by negating your character set:
(?:[^a-g])m
...which would match every m NOT preceded by any of those letters.
This is how I achieved str.split(/(?<!^)#/) for Node.js 8 (which doesn't support lookbehind):
str.split('').reverse().join('').split(/#(?!$)/).map(s => s.split('').reverse().join('')).reverse()
Works? Yes (unicode untested). Unpleasant? Yes.
following the idea of Mijoja, and drawing from the problems exposed by JasonS, i had this idea; i checked a bit but am not sure of myself, so a verification by someone more expert than me in js regex would be great :)
var re = /(?=(..|^.?)(ll))/g
// matches empty string position
// whenever this position is followed by
// a string of length equal or inferior (in case of "^")
// to "lookbehind" value
// + actual value we would want to match
, str = "Fall ball bill balll llama"
, str_done = str
, len_difference = 0
, doer = function (where_in_str, to_replace)
{
str_done = str_done.slice(0, where_in_str + len_difference)
+ "[match]"
+ str_done.slice(where_in_str + len_difference + to_replace.length)
len_difference = str_done.length - str.length
/* if str smaller:
len_difference will be positive
else will be negative
*/
} /* the actual function that would do whatever we want to do
with the matches;
this above is only an example from Jason's */
/* function input of .replace(),
only there to test the value of $behind
and if negative, call doer() with interesting parameters */
, checker = function ($match, $behind, $after, $where, $str)
{
if ($behind !== "ba")
doer
(
$where + $behind.length
, $after
/* one will choose the interesting arguments
to give to the doer, it's only an example */
)
return $match // empty string anyhow, but well
}
str.replace(re, checker)
console.log(str_done)
my personal output:
Fa[match] ball bi[match] bal[match] [match]ama
the principle is to call checker at each point in the string between any two characters, whenever that position is the starting point of:
--- any substring of the size of what is not wanted (here 'ba', thus ..) (if that size is known; otherwise it must be harder to do perhaps)
--- --- or smaller than that if it's the beginning of the string: ^.?
and, following this,
--- what is to be actually sought (here 'll').
At each call of checker, there will be a test to check if the value before ll is not what we don't want (!== 'ba'); if that's the case, we call another function, and it will have to be this one (doer) that will make the changes on str, if the purpose is this one, or more generically, that will get in input the necessary data to manually process the results of the scanning of str.
here we change the string so we needed to keep a trace of the difference of length in order to offset the locations given by replace, all calculated on str, which itself never changes.
since primitive strings are immutable, we could have used the variable str to store the result of the whole operation, but i thought the example, already complicated by the replacings, would be clearer with another variable (str_done).
i guess that in terms of performances it must be pretty harsh: all those pointless replacements of '' into '', this str.length-1 times, plus here manual replacement by doer, which means a lot of slicing...
probably in this specific above case that could be grouped, by cutting the string only once into pieces around where we want to insert [match] and .join()ing it with [match] itself.
the other thing is that i don't know how it would handle more complex cases, that is, complex values for the fake lookbehind... the length being perhaps the most problematic data to get.
and, in checker, in case of multiple possibilities of nonwanted values for $behind, we'll have to make a test on it with yet another regex (to be cached (created) outside checker is best, to avoid the same regex object to be created at each call for checker) to know whether or not it is what we seek to avoid.
hope i've been clear; if not don't hesitate, i'll try better. :)
Using your case, if you want to replace m with something, e.g. convert it to uppercase M, you can negate set in capturing group.
match ([^a-g])m, replace with $1M
"jim jam".replace(/([^a-g])m/g, "$1M")
\\jiM jam
([^a-g]) will match any char not(^) in a-g range, and store it in first capturing group, so you can access it with $1.
So we find im in jim and replace it with iM which results in jiM.
As mentioned before, JavaScript allows lookbehinds now. In older browsers you still need a workaround.
I bet my head there is no way to find a regex without lookbehind that delivers the result exactly. All you can do is working with groups. Suppose you have a regex (?<!Before)Wanted, where Wanted is the regex you want to match and Before is the regex that counts out what should not precede the match. The best you can do is negate the regex Before and use the regex NotBefore(Wanted). The desired result is the first group $1.
In your case Before=[abcdefg] which is easy to negate NotBefore=[^abcdefg]. So the regex would be [^abcdefg](m). If you need the position of Wanted, you must group NotBefore too, so that the desired result is the second group.
If matches of the Before pattern have a fixed length n, that is, if the pattern contains no repetitive tokens, you can avoid negating the Before pattern and use the regular expression (?!Before).{n}(Wanted), but still have to use the first group or use the regular expression (?!Before)(.{n})(Wanted) and use the second group. In this example, the pattern Before actually has a fixed length, namely 1, so use the regex (?![abcdefg]).(m) or (?![abcdefg])(.)(m). If you are interested in all matches, add the g flag, see my code snippet:
function TestSORegEx() {
var s = "Donald Trump doesn't like jam, but Homer Simpson does.";
var reg = /(?![abcdefg])(.{1})(m)/gm;
var out = "Matches and groups of the regex " +
"/(?![abcdefg])(.{1})(m)/gm in \ns = \"" + s + "\"";
var match = reg.exec(s);
while(match) {
var start = match.index + match[1].length;
out += "\nWhole match: " + match[0] + ", starts at: " + match.index
+ ". Desired match: " + match[2] + ", starts at: " + start + ".";
match = reg.exec(s);
}
out += "\nResulting string after statement s.replace(reg, \"$1*$2*\")\n"
+ s.replace(reg, "$1*$2*");
alert(out);
}
This effectively does it
"jim".match(/[^a-g]m/)
> ["im"]
"jam".match(/[^a-g]m/)
> null
Search and replace example
"jim jam".replace(/([^a-g])m/g, "$1M")
> "jiM jam"
Note that the negative look-behind string must be 1 character long for this to work.
I am trying to understand some code where a number is converted to a currency format. Thus, if you have 16.9 it converts to $16.90. The problem with the code is if you have an amount over $1,000, it just returns $1, an amount over $2,000 returns $2, etc. Amounts in the hundreds show up fine.
Here is the function:
var _formatCurrency = function(amount) {
return "$" + parseFloat(amount).toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, '$1,')
};
(The reason the semicolon is after the bracket is because this function is in itself a statement in another function. That function is not relevant to this discussion.)
I found out that the person who originally put the code in there found it somewhere but didn't fully understand it and didn't test this particular scenario. I myself have not dealt much with regular expressions. I am not only trying to fix it, but to understand how it is working as it is now.
Here's what I've found out. The code between the backslash after the open parenthesis and the backslash before the g is the pattern. The g means global search. The \d means digit, and the (?=\d{3})+\. appears to mean find 3 digits plus a decimal point. I'm not sure I have that right, though, because if that was correct shouldn't it ignore numbers like 5.4? That works fine. Also, I'm not sure what the '$1,' is for. It looks to me like it is supposed to be placed where the digits are, but wouldn't that change all the numbers to $1? Also, why is there a comma after the 1?
Regarding your comment
I was hoping to just edit the regex so it would work properly.
The regex you are currently using is obviously not working for you so I think you should consider alternatives even if they are not too similar, and
Trying to keep the code change as small as possible
Understandable but sometimes it is better to use a code that is a little bit bigger and MORE READABLE than to go with compact and hieroglyphical.
Back to business:
I'm assuming you are getting a string as an argument and this string is composed only of digits and may or may not have a dot before the last 1 or 2 digts. Something like
//input //intended output
1 $1.00
20 $20.00
34.2 $34.20
23.1 $23.10
62516.16 $62,516.16
15.26 $15.26
4654656 $4,654,656.00
0.3 $0.30
I will let you do a pre-check of (assumed) non-valids like 1. | 2.2. | .6 | 4.8.1 | 4.856 | etc.
Proposed solution:
var _formatCurrency = function(amount) {
amount = "$" + amount.replace(/(\d)(?=(\d{3})+(\.(\d){0,2})*$)/g, '$1,');
if(amount.indexOf('.') === -1)
return amount + '.00';
var decimals = amount.split('.')[1];
return decimals.length < 2 ? amount + '0' : amount;
};
Regex break down:
(\d): Matches one digit. Parentheses group things for referencing when needed.
(?=(\d{3})+(\.(\d){0,2})*$). Now this guy. From end to beginning:
$: Matches the end of the string. This is what allows you to match from the end instead of the beginning which is very handy for adding the commas.
(\.(\d){0,2})*: This part processes the dot and decimals. The \. matches the dot. (\d){0,2} matches 0, 1 or 2 digits (the decimals). The * implies that this whole group can be empty.
?=(\d{3})+: \d{3} matches 3 digits exactly. + means at least one occurrence. Finally ?= matches a group after the main expression without including it in the result. In this case it takes three digits at a time (from the end remember?) and leaves them out of the result for when replacing.
g: Match and replace globally, the whole string.
Replacing with $1,: This is how captured groups are referenced for replacing, in this case the wanted group is number 1. Since the pattern will match every digit in the position 3n+1 (starting from the end or the dot) and catch it in the group number 1 ((\d)), then replacing that catch with $1, will effectively add a comma after each capture.
Try it and please feedback.
Also if you haven't already you should (and SO has not provided me with a format to stress this enough) really really look into this site as suggested by Taplar
The pattern is invalid, and your understanding of the function is incorrect. This function formats a number in a standard US currency, and here is how it works:
The parseFloat() function converts a string value to a decimal number.
The toFixed(2) function rounds the decimal number to 2 digits after the decimal point.
The replace() function is used here to add the thousands spearators (i.e. a comma after every 3 digits). The pattern is incorrect, so here is a suggested fix /(\d)(?=(\d{3})+\.)/g and this is how it works:
The (\d) captures a digit.
The (?=(\d{3})+\.) is called a look-ahead and it ensures that the captured digit above has one set of 3 digits (\d{3}) or more + followed by the decimal point \. after it followed by a decimal point.
The g flag/modifier is to apply the pattern globally, that is on the entire amount.
The replacement $1, replaces the pattern with the first captured group $1, which is in our case the digit (\d) (so technically replacing the digit with itself to make sure we don't lose the digit in the replacement) followed by a comma ,. So like I said, this is just to add the thousands separator.
Here are some tests with the suggested fix. Note that it works fine with numbers and strings:
var _formatCurrency = function(amount) {
return "$" + parseFloat(amount).toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, '$1,');
};
console.log(_formatCurrency('1'));
console.log(_formatCurrency('100'));
console.log(_formatCurrency('1000'));
console.log(_formatCurrency('1000000.559'));
console.log(_formatCurrency('10000000000.559'));
console.log(_formatCurrency(1));
console.log(_formatCurrency(100));
console.log(_formatCurrency(1000));
console.log(_formatCurrency(1000000.559));
console.log(_formatCurrency(10000000000.559));
Okay, I want to apologize to everyone who answered. I did some further tracing and found out the JSON call which was bringing in the amount did in fact have a comma in it, so it is just parsing that first digit. I was looking in the wrong place in the code when I thought there was no comma in there already. I do appreciate everyone's input and hope you won't think too bad of me for not catching that before this whole exercise. If nothing else, at least I now know how that regex operates so I can make use of it in the future. Now I just have to go about removing that comma.
Have a great day!
Assuming that you are working with USD only, then this should work for you as an alternative to Regular Expressions. I have also included a few tests to verify that it is working properly.
var test1 = '16.9';
var test2 = '2000.5';
var test3 = '300000.23';
var test4 = '3000000.23';
function stringToUSD(inputString) {
const splitValues = inputString.split('.');
const wholeNumber = splitValues[0].split('')
.map(val => parseInt(val))
.reverse()
.map((val, idx, arr) => idx !== 0 && (idx + 1) % 3 === 0 && arr[idx + 1] !== undefined ? `,${val}` : val)
.reverse()
.join('');
return parseFloat(`${wholeNumber}.${splitValues[1]}`).toFixed(2);
}
console.log(stringToUSD(test1));
console.log(stringToUSD(test2));
console.log(stringToUSD(test3));
console.log(stringToUSD(test4));
Is there a way to achieve the equivalent of a negative lookbehind in JavaScript regular expressions? I need to match a string that does not start with a specific set of characters.
It seems I am unable to find a regex that does this without failing if the matched part is found at the beginning of the string. Negative lookbehinds seem to be the only answer, but JavaScript doesn't has one.
This is the regex that I would like to work, but it doesn't:
(?<!([abcdefg]))m
So it would match the 'm' in 'jim' or 'm', but not 'jam'
Since 2018, Lookbehind Assertions are part of the ECMAScript language specification.
// positive lookbehind
(?<=...)
// negative lookbehind
(?<!...)
Answer pre-2018
As Javascript supports negative lookahead, one way to do it is:
reverse the input string
match with a reversed regex
reverse and reformat the matches
const reverse = s => s.split('').reverse().join('');
const test = (stringToTests, reversedRegexp) => stringToTests
.map(reverse)
.forEach((s,i) => {
const match = reversedRegexp.test(s);
console.log(stringToTests[i], match, 'token:', match ? reverse(reversedRegexp.exec(s)[0]) : 'Ø');
});
Example 1:
Following #andrew-ensley's question:
test(['jim', 'm', 'jam'], /m(?!([abcdefg]))/)
Outputs:
jim true token: m
m true token: m
jam false token: Ø
Example 2:
Following #neaumusic comment (match max-height but not line-height, the token being height):
test(['max-height', 'line-height'], /thgieh(?!(-enil))/)
Outputs:
max-height true token: height
line-height false token: Ø
Lookbehind Assertions got accepted into the ECMAScript specification in 2018.
Positive lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<=\$)\d+\.\d*/) // Matches "9.99"
);
Negative lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<!\$)\d+\.\d*/) // Matches "8.47"
);
Platform support:
✔️ V8
✔️ Google Chrome 62.0
✔️ Microsoft Edge 79.0
✔️ Node.js 6.0 behind a flag and 9.0 without a flag
✔️ Deno (all versions)
✔️ SpiderMonkey
✔️ Mozilla Firefox 78.0
🛠️ JavaScriptCore: support in beta as of 2023-02-16: the feature has been merged
✔️ Apple Safari 16.4 (in beta as of 2023-02-16)
✔️ iOS 16.4 beta WebView (all browsers on iOS + iPadOS)
✔️ Bun 0.2.2
❌ Chakra: Microsoft was working on it but Chakra is now abandoned in favor of V8
❌ Internet Explorer
❌ Edge versions prior to 79 (the ones based on EdgeHTML+Chakra)
Let's suppose you want to find all int not preceded by unsigned :
With support for negative look-behind:
(?<!unsigned )int
Without support for negative look-behind:
((?!unsigned ).{9}|^.{0,8})int
Basically idea is to grab n preceding characters and exclude match with negative look-ahead, but also match the cases where there's no preceeding n characters. (where n is length of look-behind).
So the regex in question:
(?<!([abcdefg]))m
would translate to:
((?!([abcdefg])).|^)m
You might need to play with capturing groups to find exact spot of the string that interests you or you want to replace specific part with something else.
Mijoja's strategy works for your specific case but not in general:
js>newString = "Fall ball bill balll llama".replace(/(ba)?ll/g,
function($0,$1){ return $1?$0:"[match]";});
Fa[match] ball bi[match] balll [match]ama
Here's an example where the goal is to match a double-l but not if it is preceded by "ba". Note the word "balll" -- true lookbehind should have suppressed the first 2 l's but matched the 2nd pair. But by matching the first 2 l's and then ignoring that match as a false positive, the regexp engine proceeds from the end of that match, and ignores any characters within the false positive.
Use
newString = string.replace(/([abcdefg])?m/, function($0,$1){ return $1?$0:'m';});
You could define a non-capturing group by negating your character set:
(?:[^a-g])m
...which would match every m NOT preceded by any of those letters.
This is how I achieved str.split(/(?<!^)#/) for Node.js 8 (which doesn't support lookbehind):
str.split('').reverse().join('').split(/#(?!$)/).map(s => s.split('').reverse().join('')).reverse()
Works? Yes (unicode untested). Unpleasant? Yes.
following the idea of Mijoja, and drawing from the problems exposed by JasonS, i had this idea; i checked a bit but am not sure of myself, so a verification by someone more expert than me in js regex would be great :)
var re = /(?=(..|^.?)(ll))/g
// matches empty string position
// whenever this position is followed by
// a string of length equal or inferior (in case of "^")
// to "lookbehind" value
// + actual value we would want to match
, str = "Fall ball bill balll llama"
, str_done = str
, len_difference = 0
, doer = function (where_in_str, to_replace)
{
str_done = str_done.slice(0, where_in_str + len_difference)
+ "[match]"
+ str_done.slice(where_in_str + len_difference + to_replace.length)
len_difference = str_done.length - str.length
/* if str smaller:
len_difference will be positive
else will be negative
*/
} /* the actual function that would do whatever we want to do
with the matches;
this above is only an example from Jason's */
/* function input of .replace(),
only there to test the value of $behind
and if negative, call doer() with interesting parameters */
, checker = function ($match, $behind, $after, $where, $str)
{
if ($behind !== "ba")
doer
(
$where + $behind.length
, $after
/* one will choose the interesting arguments
to give to the doer, it's only an example */
)
return $match // empty string anyhow, but well
}
str.replace(re, checker)
console.log(str_done)
my personal output:
Fa[match] ball bi[match] bal[match] [match]ama
the principle is to call checker at each point in the string between any two characters, whenever that position is the starting point of:
--- any substring of the size of what is not wanted (here 'ba', thus ..) (if that size is known; otherwise it must be harder to do perhaps)
--- --- or smaller than that if it's the beginning of the string: ^.?
and, following this,
--- what is to be actually sought (here 'll').
At each call of checker, there will be a test to check if the value before ll is not what we don't want (!== 'ba'); if that's the case, we call another function, and it will have to be this one (doer) that will make the changes on str, if the purpose is this one, or more generically, that will get in input the necessary data to manually process the results of the scanning of str.
here we change the string so we needed to keep a trace of the difference of length in order to offset the locations given by replace, all calculated on str, which itself never changes.
since primitive strings are immutable, we could have used the variable str to store the result of the whole operation, but i thought the example, already complicated by the replacings, would be clearer with another variable (str_done).
i guess that in terms of performances it must be pretty harsh: all those pointless replacements of '' into '', this str.length-1 times, plus here manual replacement by doer, which means a lot of slicing...
probably in this specific above case that could be grouped, by cutting the string only once into pieces around where we want to insert [match] and .join()ing it with [match] itself.
the other thing is that i don't know how it would handle more complex cases, that is, complex values for the fake lookbehind... the length being perhaps the most problematic data to get.
and, in checker, in case of multiple possibilities of nonwanted values for $behind, we'll have to make a test on it with yet another regex (to be cached (created) outside checker is best, to avoid the same regex object to be created at each call for checker) to know whether or not it is what we seek to avoid.
hope i've been clear; if not don't hesitate, i'll try better. :)
Using your case, if you want to replace m with something, e.g. convert it to uppercase M, you can negate set in capturing group.
match ([^a-g])m, replace with $1M
"jim jam".replace(/([^a-g])m/g, "$1M")
\\jiM jam
([^a-g]) will match any char not(^) in a-g range, and store it in first capturing group, so you can access it with $1.
So we find im in jim and replace it with iM which results in jiM.
As mentioned before, JavaScript allows lookbehinds now. In older browsers you still need a workaround.
I bet my head there is no way to find a regex without lookbehind that delivers the result exactly. All you can do is working with groups. Suppose you have a regex (?<!Before)Wanted, where Wanted is the regex you want to match and Before is the regex that counts out what should not precede the match. The best you can do is negate the regex Before and use the regex NotBefore(Wanted). The desired result is the first group $1.
In your case Before=[abcdefg] which is easy to negate NotBefore=[^abcdefg]. So the regex would be [^abcdefg](m). If you need the position of Wanted, you must group NotBefore too, so that the desired result is the second group.
If matches of the Before pattern have a fixed length n, that is, if the pattern contains no repetitive tokens, you can avoid negating the Before pattern and use the regular expression (?!Before).{n}(Wanted), but still have to use the first group or use the regular expression (?!Before)(.{n})(Wanted) and use the second group. In this example, the pattern Before actually has a fixed length, namely 1, so use the regex (?![abcdefg]).(m) or (?![abcdefg])(.)(m). If you are interested in all matches, add the g flag, see my code snippet:
function TestSORegEx() {
var s = "Donald Trump doesn't like jam, but Homer Simpson does.";
var reg = /(?![abcdefg])(.{1})(m)/gm;
var out = "Matches and groups of the regex " +
"/(?![abcdefg])(.{1})(m)/gm in \ns = \"" + s + "\"";
var match = reg.exec(s);
while(match) {
var start = match.index + match[1].length;
out += "\nWhole match: " + match[0] + ", starts at: " + match.index
+ ". Desired match: " + match[2] + ", starts at: " + start + ".";
match = reg.exec(s);
}
out += "\nResulting string after statement s.replace(reg, \"$1*$2*\")\n"
+ s.replace(reg, "$1*$2*");
alert(out);
}
This effectively does it
"jim".match(/[^a-g]m/)
> ["im"]
"jam".match(/[^a-g]m/)
> null
Search and replace example
"jim jam".replace(/([^a-g])m/g, "$1M")
> "jiM jam"
Note that the negative look-behind string must be 1 character long for this to work.
Here is what i've got so far:
/(netscape)|(navigator)\/(\d+)(\.(\d+))?/.test(UserAgentString.toLowerCase()) ? ' netscape'+RegExp.$3+RegExp.$4 : ''
I'm trying to do several different things here.
(1). I want to match either netscape or navigator, and it must be followed by a single slash and one or more digits.
(2). It can optionally follow those digits with up to one of: one period and one or more digits.
The expression should evaluate to an empty string if (1) is not true.
The expression should return ' netscape8' if UserAgentString is Netscape/8 or Navigator/8.
The expression should return ' netscape8.4' if UserAgentString is Navigator/8.4.2.
The regex is not working. In particular (this is an edited down version for my testing, and it still doesn't work):
// in Chrome this produces ["netscape", "netscape", undefined, undefined]
(/(netscape)|(navigator)\/(\d+)/.exec("Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.7.5) Gecko/20060912 Netscape/8.1.2".toLowerCase()))
Why does the 8 not get matched? Is it supposed to show up in the third entry or the fourth?
There are a couple things that I want to figure out if they are supported. Notice how I have 5 sets of capture paren groups. group #5 \d+ is contained within group #4: \.(\d+). Is it possible to retrieve the matched groups?
Also, what happens if I specify a group like this? /(\.\d+)*/ This matches any number of "dot-number" strings contatenated together (like in a version number). What's RegExp.$1 supposed to match here?
Your "or" expression is not doing what you think.
Simplified, you're doing this:
(a)|(b)cde
Which matches either a or bcde.
Put parentheses around your "or" expression: ((a)|(b))cde and that will match either acde or bcde.
I find http://regexpal.com/ to be a very useful tool for quickly checking my regex syntax.
Regex (netscape|navigator)\/(\d+(?:\.\d+)?) will return 2 groups (if match found):
netscape or navigator
number behind the name
var m = /(netscape|navigator)\/(\d+(?:\.\d+)?)/.exec(text);
if (m != null) {
var r = m[1] + m[2];
}
(....) Creates a group. Everything inside that group is returned with that group's variable.
The following will match netscape or navigator and the first two numbers of the version separated by a period.
$1 $2
|------------------| |------------|
/(netscape|navigator)[^\/]*\/((\d+)\.(\d+))/
The final code looks like this:
/(netscape|navigator)[^\/]*\/((\d+)\.(\d+))/.test(
navigator.userAgent.toLowerCase()
) ? 'netscape'+RegExp.$2 : ''
Which will give you
netscape5.0
Check out these great tuts (there are many more):
http://perldoc.perl.org/perlrequick.html
http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
Is there a way to achieve the equivalent of a negative lookbehind in JavaScript regular expressions? I need to match a string that does not start with a specific set of characters.
It seems I am unable to find a regex that does this without failing if the matched part is found at the beginning of the string. Negative lookbehinds seem to be the only answer, but JavaScript doesn't has one.
This is the regex that I would like to work, but it doesn't:
(?<!([abcdefg]))m
So it would match the 'm' in 'jim' or 'm', but not 'jam'
Since 2018, Lookbehind Assertions are part of the ECMAScript language specification.
// positive lookbehind
(?<=...)
// negative lookbehind
(?<!...)
Answer pre-2018
As Javascript supports negative lookahead, one way to do it is:
reverse the input string
match with a reversed regex
reverse and reformat the matches
const reverse = s => s.split('').reverse().join('');
const test = (stringToTests, reversedRegexp) => stringToTests
.map(reverse)
.forEach((s,i) => {
const match = reversedRegexp.test(s);
console.log(stringToTests[i], match, 'token:', match ? reverse(reversedRegexp.exec(s)[0]) : 'Ø');
});
Example 1:
Following #andrew-ensley's question:
test(['jim', 'm', 'jam'], /m(?!([abcdefg]))/)
Outputs:
jim true token: m
m true token: m
jam false token: Ø
Example 2:
Following #neaumusic comment (match max-height but not line-height, the token being height):
test(['max-height', 'line-height'], /thgieh(?!(-enil))/)
Outputs:
max-height true token: height
line-height false token: Ø
Lookbehind Assertions got accepted into the ECMAScript specification in 2018.
Positive lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<=\$)\d+\.\d*/) // Matches "9.99"
);
Negative lookbehind usage:
console.log(
"$9.99 €8.47".match(/(?<!\$)\d+\.\d*/) // Matches "8.47"
);
Platform support:
✔️ V8
✔️ Google Chrome 62.0
✔️ Microsoft Edge 79.0
✔️ Node.js 6.0 behind a flag and 9.0 without a flag
✔️ Deno (all versions)
✔️ SpiderMonkey
✔️ Mozilla Firefox 78.0
🛠️ JavaScriptCore: support in beta as of 2023-02-16: the feature has been merged
✔️ Apple Safari 16.4 (in beta as of 2023-02-16)
✔️ iOS 16.4 beta WebView (all browsers on iOS + iPadOS)
✔️ Bun 0.2.2
❌ Chakra: Microsoft was working on it but Chakra is now abandoned in favor of V8
❌ Internet Explorer
❌ Edge versions prior to 79 (the ones based on EdgeHTML+Chakra)
Let's suppose you want to find all int not preceded by unsigned :
With support for negative look-behind:
(?<!unsigned )int
Without support for negative look-behind:
((?!unsigned ).{9}|^.{0,8})int
Basically idea is to grab n preceding characters and exclude match with negative look-ahead, but also match the cases where there's no preceeding n characters. (where n is length of look-behind).
So the regex in question:
(?<!([abcdefg]))m
would translate to:
((?!([abcdefg])).|^)m
You might need to play with capturing groups to find exact spot of the string that interests you or you want to replace specific part with something else.
Mijoja's strategy works for your specific case but not in general:
js>newString = "Fall ball bill balll llama".replace(/(ba)?ll/g,
function($0,$1){ return $1?$0:"[match]";});
Fa[match] ball bi[match] balll [match]ama
Here's an example where the goal is to match a double-l but not if it is preceded by "ba". Note the word "balll" -- true lookbehind should have suppressed the first 2 l's but matched the 2nd pair. But by matching the first 2 l's and then ignoring that match as a false positive, the regexp engine proceeds from the end of that match, and ignores any characters within the false positive.
Use
newString = string.replace(/([abcdefg])?m/, function($0,$1){ return $1?$0:'m';});
You could define a non-capturing group by negating your character set:
(?:[^a-g])m
...which would match every m NOT preceded by any of those letters.
This is how I achieved str.split(/(?<!^)#/) for Node.js 8 (which doesn't support lookbehind):
str.split('').reverse().join('').split(/#(?!$)/).map(s => s.split('').reverse().join('')).reverse()
Works? Yes (unicode untested). Unpleasant? Yes.
following the idea of Mijoja, and drawing from the problems exposed by JasonS, i had this idea; i checked a bit but am not sure of myself, so a verification by someone more expert than me in js regex would be great :)
var re = /(?=(..|^.?)(ll))/g
// matches empty string position
// whenever this position is followed by
// a string of length equal or inferior (in case of "^")
// to "lookbehind" value
// + actual value we would want to match
, str = "Fall ball bill balll llama"
, str_done = str
, len_difference = 0
, doer = function (where_in_str, to_replace)
{
str_done = str_done.slice(0, where_in_str + len_difference)
+ "[match]"
+ str_done.slice(where_in_str + len_difference + to_replace.length)
len_difference = str_done.length - str.length
/* if str smaller:
len_difference will be positive
else will be negative
*/
} /* the actual function that would do whatever we want to do
with the matches;
this above is only an example from Jason's */
/* function input of .replace(),
only there to test the value of $behind
and if negative, call doer() with interesting parameters */
, checker = function ($match, $behind, $after, $where, $str)
{
if ($behind !== "ba")
doer
(
$where + $behind.length
, $after
/* one will choose the interesting arguments
to give to the doer, it's only an example */
)
return $match // empty string anyhow, but well
}
str.replace(re, checker)
console.log(str_done)
my personal output:
Fa[match] ball bi[match] bal[match] [match]ama
the principle is to call checker at each point in the string between any two characters, whenever that position is the starting point of:
--- any substring of the size of what is not wanted (here 'ba', thus ..) (if that size is known; otherwise it must be harder to do perhaps)
--- --- or smaller than that if it's the beginning of the string: ^.?
and, following this,
--- what is to be actually sought (here 'll').
At each call of checker, there will be a test to check if the value before ll is not what we don't want (!== 'ba'); if that's the case, we call another function, and it will have to be this one (doer) that will make the changes on str, if the purpose is this one, or more generically, that will get in input the necessary data to manually process the results of the scanning of str.
here we change the string so we needed to keep a trace of the difference of length in order to offset the locations given by replace, all calculated on str, which itself never changes.
since primitive strings are immutable, we could have used the variable str to store the result of the whole operation, but i thought the example, already complicated by the replacings, would be clearer with another variable (str_done).
i guess that in terms of performances it must be pretty harsh: all those pointless replacements of '' into '', this str.length-1 times, plus here manual replacement by doer, which means a lot of slicing...
probably in this specific above case that could be grouped, by cutting the string only once into pieces around where we want to insert [match] and .join()ing it with [match] itself.
the other thing is that i don't know how it would handle more complex cases, that is, complex values for the fake lookbehind... the length being perhaps the most problematic data to get.
and, in checker, in case of multiple possibilities of nonwanted values for $behind, we'll have to make a test on it with yet another regex (to be cached (created) outside checker is best, to avoid the same regex object to be created at each call for checker) to know whether or not it is what we seek to avoid.
hope i've been clear; if not don't hesitate, i'll try better. :)
Using your case, if you want to replace m with something, e.g. convert it to uppercase M, you can negate set in capturing group.
match ([^a-g])m, replace with $1M
"jim jam".replace(/([^a-g])m/g, "$1M")
\\jiM jam
([^a-g]) will match any char not(^) in a-g range, and store it in first capturing group, so you can access it with $1.
So we find im in jim and replace it with iM which results in jiM.
As mentioned before, JavaScript allows lookbehinds now. In older browsers you still need a workaround.
I bet my head there is no way to find a regex without lookbehind that delivers the result exactly. All you can do is working with groups. Suppose you have a regex (?<!Before)Wanted, where Wanted is the regex you want to match and Before is the regex that counts out what should not precede the match. The best you can do is negate the regex Before and use the regex NotBefore(Wanted). The desired result is the first group $1.
In your case Before=[abcdefg] which is easy to negate NotBefore=[^abcdefg]. So the regex would be [^abcdefg](m). If you need the position of Wanted, you must group NotBefore too, so that the desired result is the second group.
If matches of the Before pattern have a fixed length n, that is, if the pattern contains no repetitive tokens, you can avoid negating the Before pattern and use the regular expression (?!Before).{n}(Wanted), but still have to use the first group or use the regular expression (?!Before)(.{n})(Wanted) and use the second group. In this example, the pattern Before actually has a fixed length, namely 1, so use the regex (?![abcdefg]).(m) or (?![abcdefg])(.)(m). If you are interested in all matches, add the g flag, see my code snippet:
function TestSORegEx() {
var s = "Donald Trump doesn't like jam, but Homer Simpson does.";
var reg = /(?![abcdefg])(.{1})(m)/gm;
var out = "Matches and groups of the regex " +
"/(?![abcdefg])(.{1})(m)/gm in \ns = \"" + s + "\"";
var match = reg.exec(s);
while(match) {
var start = match.index + match[1].length;
out += "\nWhole match: " + match[0] + ", starts at: " + match.index
+ ". Desired match: " + match[2] + ", starts at: " + start + ".";
match = reg.exec(s);
}
out += "\nResulting string after statement s.replace(reg, \"$1*$2*\")\n"
+ s.replace(reg, "$1*$2*");
alert(out);
}
This effectively does it
"jim".match(/[^a-g]m/)
> ["im"]
"jam".match(/[^a-g]m/)
> null
Search and replace example
"jim jam".replace(/([^a-g])m/g, "$1M")
> "jiM jam"
Note that the negative look-behind string must be 1 character long for this to work.