I have read recently about JavaScript regular expressions, but I am confused.
The author says that it is necessary to include the caret (^) and dollar symbol ($) at the beginning and end of the all regular expressions declarations.
Why are they needed?
Javascript RegExp() allows you to specify a multi-line mode (m) which changes the behavior of ^ and $.
^ represents the start of the current line in multi-line mode, otherwise the start of the string
$ represents the end of the current line in multi-line mode, otherwise the end of the string
For example: this allows you to match something like semicolons at the end of a line where the next line starts with "var" /;$\n\s*var/m
Fast regexen also need an "anchor" point, somewhere to start it's search somewhere in the string. These characters tell the Regex engine where to start looking and generally reduce the number of backtracks, making your Regex much, much faster in many cases.
NOTE: This knowledge came from Nicolas Zakas's High Performance Javascript
Conclusion: You should use them!
^ represents the start of the input string.
$ represents the end.
You don't actually have to use them at the start and end. You can use em anywhere =) Regex is fun (and confusing). They don't represent a character. They represent the start and end.
This is a very good website
They match the start of the string (^) and end of the string ('$').
You should use them when matching strings at the start or end of the string. I wouldn't say you have to use them, however.
I have tested these.
1. /^a/ matches abb, ab but not ba, bab, bba.
2. /a/ matches abb, ab and ba, bab, bba.
I think that /^a/ matches such strings starting a.
/a/ matches such strings contains a.
Similar to /^a/, /a$/ matches ba, a, but not ab, bab.
Refer http://www.regular-expressions.info/anchors.html .
If you notify wrong(or strange) sentence in above or this to me, I would thank you.
^ anchors the beginning of the RE at the start of the test string, and $ anchors the end of the RE at the end of the test string. If that's what you want, go for it! However, if you're using REs of the form ^.*theRealRE.*$ then you might want to consider dropping the anchors and just using the core of the RE on its own.
Some languages force REs to be anchored at both ends by default.
Related
I would like to find a regular expression that will match a row, which contains a certain word (or character) and is the last line of the string.
Any ideas on how I could do this?
The idea is to use the fact that the dot doesn't match newlines.
You can use this kind of pattern:
.*?TARGET.*$
or to isolate the target:
TARGET(?=.*$)
Notices:
You have to take care that the multiline mode (the m modifier for the most regex engines) isn't activated, otherwise $ will match the end of a line (and not the end of the string in particular).
If available, prefer to use the \z anchor instead of $ because in Perl compatible regex engine, $ succeeds also before a trailing newline sequence (however you can also take advantage of this flexibility):
.*?TARGET.*\z
I am trying to write a script to detect variable types and have been having an issue with my regular expression.
if(/\$\b([a-zA-Z]|_ )(\w)*\b /x && !/ /)
is what I am using to detect a scalar. The problem right now though is that \b \b doesn't seem to be working with the special characters (!##$, etc). For example it would count $var### as a valid name. Any ideas?
The regex you have is correct. all you need to do is to anchor it to the start and end of the string
^\$\b([a-zA-Z]|_ )(\w)*\b$
example http://regex101.com/r/uD1eR7/1
Changes made
^ anchors the regex at the start of the string
$ anchors the regex at the end of the string
Note you can also move the underscore _ into the character class and remove the word boundaries as it does not give extra advantage
^\$[a-zA-Z_]\w*$
I want to match the following pattern:
Exxxx49 (where x is a digit 0-9)
For example, E123449abcdefgh, abcdefE123449987654321 are both valid. I.e., I need to match the pattern anywhere in a string.
I am using:
^*E[0-9]{4}49*$
But it only matches E123449.
How can I allow any amount of characters in front or after the pattern?
Remove the ^ and $ to search anywhere in the string.
In your case the * are probably not what you intended; E[0-9]{4}49 should suffice. This will find an E, followed by four digits, followed by a 4 and a 9, anywhere in the string.
I would go for
^.*E[0-9]{4}49.*$
EDIT:
since it fullfills all requirements state by OP.
"[match] Exxxx49 (where x is digit 0-9)"
"allow for any amount of characters in front or after pattern"
It will match
^.* everything from, including the beginning of the line
E[0-9]{4}49 the requested pattern
.*$ everthing after the pattern, including the the end of the line
Your original regex had a regex pattern syntax error at the first *. Fix it and change it to this:
.*E\d{4}49.*
This pattern is for matching in engines (most engines) that are anchored, like Java. Since you forgot to specify a language.
.* matches any number of sequences. As it surrounds the match, this will match the entire string as long as this match is located in the string.
Here is a regex demo!
Just simply use this:
E[0-9]{4}49
How do I allow for any amount of characters in front or after pattern? but it only matches E123449
Use global flag /E\d{4}49/g if supported by the language
OR
Try with capturing groups (E\d{4}49)+ that is grouped by enclosing inside parenthesis (...)
Here is online demo
I'm having trouble doing simple things with regex in dot net.
Suppose I want to find all lines that contain the word "pizza". I would think I would do the following:
^ .* pizza .* $
The idea is the first character indicates the start of a line, the dollar sign indicates the end of the line, and the dot-star indicates any number of characters.
This doesn't seem to work.
Then I tried something else that doesn't work either. I thought I would find all routines in my visual basic project that start with "Sub Page_Load" and end with "End Sub". I did a search for:
Sub Page_Load .* End Sub
But this found pretty much EVERY subroutine in the project.
In other words, it didn't limit itself to the Page_Load sub.
So I thought I'd be smart and notice that every End Sub is at the end of a line, so all I have to do is put a $ after it like this:
Sub Page_Load .* End Sub$
But that finds absolutely zero strings.
So what am I doing wrong? (one note, I put extra blanks around .* here so you can see it, but normally the blanks would not be there.
you may need non-greedy approach. try this:
^.*?pizza.*$
So, now complete new answer.
Search for the word "pizza" (not "pizzas")
If you have a Multiline string and want to find a single row, you need to use the Option [Multiline][1]. That changes the behaviour of the anchors ^ and $ to match the start and the end of the row.
To ensure to match only the complete word "pizza" and no partial match, use word boundaries
If you don't use the Singleline option, you don't need to worry about greediness
So your regex would be:
Regex optionRegex = new Regex(#"^.*\bpizza\b.*$", RegexOptions.Multiline);
For the Sub Page_Load.*End Sub thing, you need to match more than one line:
Use the single line option, to allow the . match also newline characters.
You need ungreedy matching behaviour of the quantifier
So your regex would be:
Regex optionRegex = new Regex(#"Sub Page_Load.*?End Sub", RegexOptions.Singleline);
I'm trying to come up with a regex that helps me validate a Blood Group field - which should accept only A[+-], B[+-], AB[+-] and O[+-].
Here's the regex I came up with (and tested using Regex Tester):
[A|B|AB|O][\+|\-]
Now this pattern successfully matches A,B,O[+-] but fails against AB[+-].
Can anyone please suggest a regex that'll serve my purpose?
Thanks,
m^e
Try:
(A|B|AB|O)[+-]
Using square brackets defines a character class, which can only be a single character. The parentheses create a grouping which allows it to do what you want. You also don't need to escape the +- in the character class, as they don't have their regexy meaning inside of it.
As you mentioned in the comments, if it is a string you want to match against that has the exact values you are looking for, you might want to do this:
^(A|B|AB|O)[+-]$
Without the start of string and end of string anchors, things like "helloAB+asdads" would match.
The brackets [] denote a character class, meaning "any of the characters herein". You want the parentheses () for grouping:
(A|B|AB|0)(\+|-)
When you are building an alternation (e.g. (A|B|AB|O)), you should be careful with the ordering of the elements. Many regex engines will stop at the first alternate that matches (rather than the longest). If it weren't for the [-+] forcing a backtrack, (A|B|AB|O)[-+] would not work for "AB+". It is probably better to say (AB|A|B|O)[-+] (but you should check the docs for your regex engine).
Also, if you do not intend to capture the antigen for latter use, you should you use the non-capturing grouping parentheses: (?:AB|A|B|O)[-+].
Furthermore, if you want to ensure that the only thing in the string is a blood type then you need anchors to prevent it from matching only part of the string: ^(?:AB|A|B|O)[-+]$. A quick note on anchors, Depending on your regex engine, ^ may match the beginning of a line rather than the beginning of the string if you pass it a multiline-match option. Similarly, $ may match the end of a line rather than the end of a string. For this reason there are three other anchors in common (but not %100) usage: \A, \Z, and \z. If your regex engine supports them, \A always matches the start of the string, \Z matches the end of the string or a newline just before the end of the string, and \z matches only the send of the string.
For case insensitive within html pattern attribute you may try this
([AaBbOo]|[Aa][Bb])[\+-]
<input type="text" maxlength="3" pattern="([AaBbOo]|[Aa][Bb])[\+-]" required />
^(A|B|AB|O)[+-]?$
This will produce the correct out put.