I'm just starting to learn regular expressions, and one of the questions was to match the different types of numbers. The ones that I needed to match are in below:
my regex: -?\d+,?\d+\.?e?\d+
3.14529
-255.34
128
1.9e10
123,340.00
however, from my regular expression, I failed to meet the first one and the fourth one. I saw the solution but I did not quite understand why it uses brackets. Can anyone explain? Thank you!
Your regex does not allow digits to follow a literal dot when you only have one digit preceding it. This is because you have twice a \d+ before matching the dot. In general, you have three mandatory \d+ in there, so you cannot match anything with less than three digits.
I would suggest this regex:
^-?\d{1,3}(,\d{3})*(\.\d+)?(e\d+)?$
Related
I'm writing a simple, small app that allows me to share information. I have a question on using regx to validate email address.
I'm kind learning on my own. But when it comes to real-world examples, such that strings that can be validated with regular expressions, I'm kind stuck.
Exercise:
Untangle the following regular expression that validates an email address:
[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*#(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?
It looks like a jumble of characters.
Can someone please explain to me how does this work?
I try to use this online resources by by Jan Goyvaerts.
Any help I will appreciate it.
First of all, there is a good thread about totally the same thing:
Using a regular expression to validate an email address
Then, below there is the explanation of your regular expression:
[a-z0-9!#$%&'*+/=?^_`{|}~-]+
- The square brackets represent the symbol class, containing all the symbols which are in the square brackets. The plus sign ('+') is a quantifier, which means that the sequence of symbols, represented by this symbol class must be at least one character long.
Also, the '+' is greedy, and, therefore, this part of the pattern will match the symbol sequence of the maximal possible length.
Talking about the square brackets contents, 'a-z' means any symbol in a range, which could be described mathematically as [a, z], and '0-9' is similar. All the other symbols are just symbols in this case.
(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*
- In Regular Expressions, the brackets represent grouping, and the asterisk ('*') is a greedy quantifier, which means "occurs zero or more times". So here we are not sure if we are going to find the brackets content, but we do not rule out the possibility.
Then, inside the brackets, we see the ?: character combination, which, being put inside brackets tells us that the symbol group inside should not be captured as a sub-string for the further reference.
Going further, \. means just a usual dot (see Escape sequence), since a dot symbol is a meta-symbol in Regex.
After the dot we see again the character of symbols, explained above.
#(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+
- Here we see the at symbol ('#'), which is just a symbol here, then there is a non-capturing symbol group, which will occur one or more times (because of + after it), and which includes a single symbol of [a-z0-9] class and another non-capturing group of symbols, which contents you can totally describe using my explanations above except for a question mark sign ('?'), which means "either once or not at all" in this context (i.e. if it is used as a quantifier).
[a-z0-9](?:[a-z0-9-]*[a-z0-9])?
- This last part is similar to what is found in a symbol group, explained above, so I believe you have now enough information to understand it.
More on quantifier types here: Greedy vs. Reluctant vs. Possessive Quantifiers.
A good Regular Expressions reference: Regular Expression Language - Quick Reference
Some information on capturing in Regular Expressions: Regex Tutorial - Parentheses for Grouping and Capturing
About special characters: Regex Tutorial - Literal Characters and Special Characters
Regex statements can be a fun yet tricky to follow. There are 5 parts to this statement.
One valid characters for a username
[a-z0-9!#$%&'*+/=?^_`{|}~-]+
check for a single '.' and any additional amount of characters
(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*
The '#' symbol
Valid second / lower level domain
(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+
A valid top level domain
[a-z0-9](?:[a-z0-9-]*[a-z0-9])?
I recommend http://www.ultrapico.com/expresso.htm. It will break the statement down for you.
I've found a remarkable tool for visualizing regular expressions here: http://regexper.com
It shows me that your regular expression breaks down like this. Hopefully this helps explain it.
[a-z0-9!#$%&'*+/=?^_`{|}~-]+
This looks for at least one of of the characters given here (a-z, 0-9, and those special characters).
(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)
This looks for the same as above, but only when it stands after a dot. This part is optional and can be repeated indefinitely. It prevents dots at the end of the name.
#
Matches the # symbol
(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+
This matches a-z, 0-9 ending with a dot and optional - in the middle ending with a dot. This has to be matched at least once.
[a-z0-9](?:[a-z0-9-]*[a-z0-9])?
This looks for a-z or 0-9, optionally followed by a-z, 0-9, -, but it cant end with a - again.
Two Suggestions I have for you.
Escaping special characters is messy. 2. Email addresses are complicated. I probably recommend you to study this post if you are really interested. Please check out this other posts: Validation in Regex and Regex Help.
See this answer. The problem is probably too difficult to solve. Two problems you have here. 1. RegEx are not easy. 2. Escaping special characters is messy. Finally, Email addresses are complicated. I probably recommend you to study this post if you are really interested.
I have been solving old question from stack so that I can improve my regex knowledge. As I have a basic knowledge of regex, most of them were easy but this question regex problem is tough.
It asks for a regex that extracts from this kind of string ou=persons,ou=(.*),dc=company,dc=org the last string immediately preceded by a comma not followed by (.*). In the last case, this should give dc=company,dc=org.
The solution is (?<=,(?!.*\Q(.*)\E)).* but I cannot understand its flow. I understood (?!.*\Q(.*)\E) portion but other are still mystery to me. Specially ?<= which is a positive look-behind. Does it search from end of string? Can anyone explain it to me like I am a 7 year old kid — and please http://regex101.com/ is not helping.
The RegEx (?<=,(?!.*\Q(.*)\E)).* look-behind potion works like this:
Start at the beginning of the string at first character.
Can we match the the thing we are looking for? ,(?!.*\Q(.*)\E)
If we can't: Move forward one character, Go To 2. and check match again.
If a match is found: Capture all the remaining characters until we can't find any .* (or generally then try the matching the remaining RegEx).
For a more wordly explaination consider reading Lookahead and Lookbehind Zero-Length Assertions.
A lookbehind allows you to specify a context just before the actual match.
You can say ,(dc=) and only return the capture group, or ,\Kdc=, or (?<=,)dc= to return the match on dc= but require that the comma is present just before the match.
The facility also allows for multiple lookbehinds, so you could do (?<=a.*)(?<=b.*)c to match c only if it is preceded by both a and b somewhere in the input.
A lookbehind is basically syntactic sugar, in that you can usually rephrase your conditions using some other regex construct. It can be really handy when you have multiple unanchored constraints, like in the last example
I am looking for a regex to match a string like this: 1,2,4-6,9,11-13,20.
Restrictions:
Only numbers, comma and hyphen are allowed
no spaces are allowed
Your question is rather vague. I would suggest improving it, or reading a tutorial on regexes.
Based on your restriction your regex is /^[-\d,]*$/ but I am quite sure that this is not what you want.
You should provide examples of input, output, the regex flavor you will be using and last but not least your attempts to solve the problem.
I am guessing you want to match comma seprated lists of positive integers or positive integer ranges. \d+ matches integers, to allow ranges, you'd use \d+(-\d+)?.
So, the regex
\d+(-\d+)?(,\d+(-\d+)?)*
would do.
I'm trying to parse a document that has reference numbers littered throughout it.
Text text text {4:2} more incredible text {4:3} much later on
{222:115} and yet some more text.
The references will always be wrapped in brackets, and there will always be a colon between the two. I wrote an expression to find them.
{[0-9]:[0-9]}
However, this obviously fails the moment you come across a two or three digit number, and I'm having trouble figuring out what that should be. There won't ever be more than 3 digits {999:999} is the maximum size to deal with.
Anybody have an idea of a proper expression for handling this?
{[0-9]+:[0-9]+}
try adding plus(es)
What regex engine are you using? Most of them will support the following expression:
\{\d+:\d+\}
The \d is actually shorthand for [0-9], but the important part is the addition of + which means "one or more".
Try this:
{[0-9]{1,3}:[0-9]{1,3}}
The {1,3} means "match between 1 and 3 of the preceding characters".
You can specify how many times you want the previous item to match by using {min,max}.
{[0-9]{1,3}:[0-9]{1,3}}
Also, you can use \d for digits instead of [0-9] for most regex flavors:
{\d{1,3}:\d{1,3}}
You may also want to consider escaping the outer { and }, just to make it clear that they are not part of a repetition definition.
<A "SystemTemperatureOutOfSpec" >
What should be the regular expression for parsing the string inside "". In the above sample it is 'SystemTemperatureOutOfSpec'
In JavaScript, this regexp:
/"([^"]*)"/
ex.
> /"([^"]*)"/.exec('<A "SystemTemperatureOutOfSpec" >')[1]
"SystemTemperatureOutOfSpec"
Similar patterns should work in a bunch of other programming languages.
try this
string Exp = "\"!\"";
I am not sure I understand your question well but if you need to match everything between double quotes, here it is: /(?<=").*?(?=")/s
(?<=<A\s")(?<content>.*)(?="\s>)
Regular expressions don't get much easier than this, so you should be able to solve it by yourself. Here's how you go about doing that:
The first step is to try to define as precisely as possible what you want to find. Let's start with this: you want to find a quote, followed by some number of characters other than a quote, followed by a quote. Is that correct? If so, our pattern has three parts: "a quote", "some characters other than a quote", and "a quote".
Now all we need to do is figure out what the regular expressions for those patterns are.
A quote
For "a quote", the pattern is literally ". Regular expressions have special characters which you have to be aware of (*, ., etc). Anything that's not a special character matches itself, and " is one of those characters. For a complete list of special characters for your language, see the documentation.
Characters other than a quote
So now the question is, how do we match "characters other than a quote"? That sounds like a range. A range is square brackets with a list of allowable characters. If the list begins with ^ it means it is a list of not-allowed characters. We want any characters other than a quote, so that means [^"].
"Some"
That range just means any one of the characters in the range, but we want "some". "Some" usually means either zero-or-more, or one-or-more. You can place * after a part of an expression to mean zero-or-more of that part. Likewise, use + to mean one-or-more (and ? means zero-or-one). There are a few other variations, but that's enough for this problem.
So, "some characters other than a quote" is the range [^"] (any character other than a quote) followed by * (zero-or-more). Thus, [^"]*
Putting it all together
This is the easy part: just combine all the pieces. A quote, followed by some characters other than a quote, followed by a quote, is "[^"]*".
Capturing the interesting part
The pattern we have will now match your string. What you want, however, is just the part inside the quotes. For that you need a "capturing group", which is denoted by parenthesis. To capture a part of a regular expression, put it in parenthesis. So, if we want to capture everything but the beginning and ending quote, the pattern becomes "([^"]*)".
And that's how you learn regular expressions. Break your problem down into a precise statement composed of short sequences of characters, figure out the regular expression for each sequence, then put it all together.
The pattern in this answer may not actually be the perfect answer for you. There are some edge cases to worry about. For example, you may only want to match a quote following a non-word character, or only quotes at the beginning or end of a word. That's all possible, but is highly dependent on your exact problem. Figuring out how to do that is just as easy though -- decide what you want, then look at the documentation to see how to accomplish that.
Spend one day practicing on regular expressions and you'll never have to ask anyone for help with regular expressions for the rest of your career. They aren't hard, but they do require concentrated study.
Are you sure you need regular expression matching here? Looking at your "string" you might be better off using a Xml parser?