Regular expression for floating point numbers

Question

I have a task to match floating point numbers  I have written the following regular expression for it       -    0-9      0-9     But  it returns an error    Invalid escape sequence  valid ones are   b   t   n   f   r                 As per my knowledge  we need to use an escape character for the   also  Please correct me where I am wrong

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In c notation  float number can occur in following shapes    123 123  123 24  24 2e-2   2   10 pow -2   2   0 1 4E 4   4   10 pow 4   4   10 000   For creating float regular expresion  I will first create  int regular expresion variable       1-9  0-9    0  will be int   Now  I will write small chunks of float regular expresion - solution is to concat those chunks with or simbol       Chunks   -     -   int   satysfies case 1 -     -   int      0-9     satysfies cases 2 and 3 -      0-9    satysfies case 4 -    -   int  eE   -   int   satysfies cases 5 and 6   Final solution  concanating small chunks        -   int       -   int      0-9         0-9       -   int  eE   -   int

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0-9  1 2         0-9  1 1       This will match    1 2 12 3 1 2 12 3

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for javascript   const test   new RegExp         0-9  0          0-9  0 2       g      Which would work for  1 23 1234 22 0 0 12 12  You can change the parts in the    to get different results in decimal length and front of the decimal as well  This is used in inputs for entering in number and checking every input as you type only allowing what passes

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This is simple  you have used Java and you ought to use     instead of     search for character escaping in Java

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-     1-9  0-9     0        0-9         -   - optional leading sign     1-9  0-9     0   - integer without leading zero  including single zero        0-9     - optional fractional part

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what you need is     -     0-9      0-9       I escaped the     and  -  sign and also grouped the decimal with its following digits since something like  1   is not a valid number   The changes will allow you to match integers and floats  for example   0  1 -2 0 2 23442

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TL DR  Use     instead of    and  0-9  instead of  d to avoid escaping issues in some languages  like Java    Thanks to the nameless one for originally recognizing this    One relatively simple pattern for matching a floating point number is    -    0-9        0-9     This will match    123 123 456  456   See a working example  If you also want to match 123   a period with no decimal part   then you ll need a slightly longer expression     -    0-9       0-9         0-9      See pkeller s answer for a fuller explanation of this pattern  If you want to include non-decimal numbers  such as hex and octal  see my answer to How do I identify if a string is a number    If you want to validate that an input is a number  rather than finding a number within the input   then you should surround the pattern with   and    like so      -    0-9       0-9         0-9       Irregular Regular Expressions   Regular expressions   as implemented in most modern languages  APIs  frameworks  libraries  etc   are based on a concept developed in formal language theory  However  software engineers have added many extensions that take these implementations far beyond the formal definition  So  while most regular expression engines resemble one another  there is actually no standard  For this reason  a lot depends on what language  API  framework or library you are using     Incidentally  to help reduce confusion  many have taken to using  regex  or  regexp  to describe these enhanced matching languages  See Is a Regex the Same as a Regular Expression  at RexEgg com for more information    That said  most regex engines  actually  all of them  as far as I know  would accept     Most likely  there s an issue with escaping   The Trouble with Escaping  Some languages have built-in support for regexes  such as JavaScript  For those languages that don t  escaping can be a problem   This is because you are basically coding in a language within a language  Java  for example  uses   as an escape character within it s strings  so if you want to place a literal backslash character within a string  you must escape it      creates a single character string      String x           However  regexes also use the   character for escaping  so if you want to match a literal   character  you must escape it for the regexe engine  and then escape it again for Java      Creates a two-character string          When used as a regex pattern  will match a single character      String regexPattern             In your case  you have probably not escaped the backslash character in the language you are programming in      will most likely result in an  Illegal escape character  error String wrongPattern            will result in the string      String correctPattern            All this escaping can get very confusing  If the language you are working with supports raw strings  then you should use those to cut down on the number of backslashes  but not all languages do  most notably  Java   Fortunately  there s an alternative that will work some of the time   String correctPattern            For a regex engine     and     mean exactly the same thing  Note that this doesn t work in every case  like newline    n   open square bracket       and backslash       or         A Note about Matching Numbers   Hint  It s harder than you think   Matching a number is one of those things you d think is quite easy with regex  but it s actually pretty tricky  Let s take a look at your approach  piece by piece    -      Match an optional - or     0-9     Match 0 or more sequential digits        Match an optional     0-9     Match 0 or more sequential digits  First  we can clean up this expression a bit by using a character class shorthand for the digits  note that this is also susceptible to the escaping issue mentioned above     0-9     d  I m going to use  d below  but keep in mind that it means the same thing as  0-9    Well  actually  in some engines  d will match digits from all scripts  so it ll match more than  0-9  will  but that s probably not significant in your case    Now  if you look at this carefully  you ll realize that every single part of your pattern is optional  This pattern can match a 0-length string  a string composed only of   or -  or  a string composed only of a    This is probably not what you ve intended   To fix this  it s helpful to start by  anchoring  your regex with the bare-minimum required string  probably a single digit    d    Now we want to add the decimal part  but it doesn t go where you think it might    d     d     This isn t quite correct       This will still match values like 123   Worse  it s got a tinge of evil about it  The period is optional  meaning that you ve got two repeated classes side-by-side   d  and  d    This can actually be dangerous if used in just the wrong way  opening your system up to DoS attacks   To fix this  rather than treating the period as optional  we need to treat it as required  to separate the repeated character classes  and instead make the entire decimal portion optional    d     d       Better  But         This is looking better now  We require a period between the first sequence of digits and the second  but there s a fatal flaw  we can t match  123 because a leading digit is now required   This is actually pretty easy to fix  Instead of making the  decimal  portion of the number optional  we need to look at it as a sequence of characters  1 or more numbers that may be prefixed by a   that may be prefixed by 0 or more numbers     d      d    Now we just add the sign     -    d      d    Of course  those slashes are pretty annoying in Java  so we can substitute in our long-form character classes     -    0-9        0-9     Matching versus Validating  This has come up in the comments a couple times  so I m adding an addendum on matching versus validating   The goal of matching is to find some content within the input  the  needle in a haystack    The goal of validating is to ensure that the input is in an expected format   Regexes  by their nature  only match text  Given some input  they will either find some matching text or they will not  However  by  snapping  an expression to the beginning and ending of the input with anchor tags    and     we can ensure that no match is found unless the entire input matches the expression  effectively using regexes to validate   The regex described above    -    0-9        0-9    will match one or more numbers within a target string  So given the input   apple 1 34 pear 7 98 version 1 2 3 4   The regex will match 1 34  7 98  1 2   3 and  4   To validate that a given input is a number and nothing but a number   snap  the expression to the start and end of the input by wrapping it in anchor tags      -    0-9        0-9      This will only find a match if the entire input is a floating point number  and will not find a match if the input contains additional characters  So  given the input 1 2  a match will be found  but given apple 1 2 pear no matches will be found   Note that some regex engines have a validate  isMatch or similar function  which essentially does what I ve described automatically  returning true if a match is found and false if no match is found  Also keep in mind that some engines allow you to set flags which change the definition of   and    matching the beginning end of a line rather than the beginning end of the entire input  This is typically not the default  but be on the lookout for these flags

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This one worked for me     P lt value gt  -    d    d   -    d     You can also use this one  without named parameter      -    d    d   -    d     Use some online regex tester to test it  e g  regex101

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In C   using the regex library   The answer would go about like this     0-9    0-9        0-9     Notice that I don t take the sign symbol  if you wanted it with the sign symbol it would go about this      -    0-9        0-9     This also separates a regular number or a decimal number

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I want to match what most languages consider valid numbers  integer and floats      5     -5   1 0     1       1     -1      - 1   0 45326e 04    666999e-05    0 2e-3    -33 e-1    Notes    preceding sign of number   -  or      is optional  -1   and  - 1  are valid but     and  -   are invalid   1e3  is valid  but   e3  and  e3  are invalid   In order to support both  1   and   1  we need an OR operator       in order to make sure we exclude     from matching     -     - sing is optional since   means 0 or 1 matches    since we have 2 sub expressions we need to put them in parenthesis   d      d    e  -   d    This is for numbers starting with a digit    separates sub expressions      d  e  -   d    this is for numbers starting with        end of expressions   For numbers starting with           first character is dot  inside brackets or else it is a wildcard character    d  one or more digits   e  -   d    this is an optional  0 or 1 matches due to ending      scientific notation   For numbers starting with a digit    d  one or more digits       d    optionally we can have a dot character an zero or more digits after it    e  -   d    this is an optional scientific notation   Scientific notation   e literal that specifies exponent    -   optional exponent sign    d  one or more digits  All of those combined     -    d      d    e  -   d        d  e  -   d       To accept E as well     -    d      d     eE   -   d        d   eE   -   d        Test cases

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For those who searching a regex which would validate an entire input that should be a signed float point number on every single character typed by a user  I e  a sign goes first  should match and be valid   then all the digits  still match and valid  and its optional decimal part  In JS  we use onkeydown oninput event to do that   the following regex     -   0-9        0-9

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-   0-9    0-9     Try this solution

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I don t think that any of the answers on this page at the time of writing are correct  also many other suggestions elsewhere on SO are wrong too   The complication is that you have to match all of the following possibilities    No decimal point  i e  an integer value  Digits both before and after the decimal point  e g  0 35   22 165  Digits before the decimal point only  e g  0    1234   Digits after the decimal point only  e g   0    5678    At the same time  you must ensure that there is at least one digit somewhere  i e  the following are not allowed    a decimal point on its own a signed decimal point with no digits  i e     or -     or - on their own an empty string   This seems tricky at first  but one way of finding inspiration is to look at the OpenJDK source for the java lang Double valueOf String  method  start at http   hg openjdk java net jdk8 jdk8 jdk  click  browse   navigate down  src share classes java lang  and find the Double class   The long regex that this class contains caters for various possibilities that the OP probably didn t have in mind  but ignoring for simplicity the parts of it that deal with NaN  infinity  Hexadecimal notation and exponents  and using  d rather than the POSIX notation for a single digit  I can reduce the important parts of the regex for a signed floating point number with no exponent to     -     d     d       d     I don t think that there is a way of avoiding the             construction without allowing something that contains no digits  or forbidding one of the possibilities that has no digits before the decimal point or no digits after it   Obviously in practice you will need to cater for trailing or preceding whitespace  either in the regex itself or in the code that uses it

[regex] Regular expression for floating point numbers

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