Unfortunately, the currently accepted answer is flawed (it will match "1a5", but it won't match ".5"). So I decided to write my own Regex, as well as a cheap Perl script to test it.
Requirements
The Regex will match an integer or float, with a preceding +/- sign. I don't care for E notation, so I ignored it.
nhahtdh mentioned "(5.) and (.6)": "5." does not look right to me, the trailing period has no purpose. However, ".6" is indeed valid, as the leading period indicates a positive value < 1 (i.e., it's short for "0.6").
The initial (?=.) is to make sure a blank string won't match.
As a bonus, I used nested groups, so that you can easily extract the different parts of the number:
- $1 = The whole number
- $2 = Integer part
- $3 = Fractional part with leading period
- $4 = Fractional part
Regex
^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$
Test
$ perl -e 'use strict; use warnings; my $a = "1.0"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<1.0>
Integer:<1>
Fraction:<0>
$ perl -e 'use strict; use warnings; my $a = "1"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<1>
Integer:<1>
No fraction
$ perl -e 'use strict; use warnings; my $a = "9.9"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<9.9>
Integer:<9>
Fraction:<9>
$ perl -e 'use strict; use warnings; my $a = "10000000"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<10000000>
Integer:<10000000>
No fraction
$ perl -e 'use strict; use warnings; my $a = "+15"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<+15>
Integer:<15>
No fraction
$ perl -e 'use strict; use warnings; my $a = "1,5"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
$ perl -e 'use strict; use warnings; my $a = "a"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
$ perl -e 'use strict; use warnings; my $a = "1 5"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
$ perl -e 'use strict; use warnings; my $a = "1+5"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
$ perl -e 'use strict; use warnings; my $a = "5."; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
$ perl -e 'use strict; use warnings; my $a = ".6"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<.6>
Integer:<>
Fraction:<6>
$ perl -e 'use strict; use warnings; my $a = ".5"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
MATCH
Float:<.5>
Integer:<>
Fraction:<5>
$ perl -e 'use strict; use warnings; my $a = "1a5"; if ($a =~ /^(?=.)([+-]?([0-9]*)(\.([0-9]+))?)$/) { print "MATCH\nFloat:<$1>\n"; print defined $2 ? "Integer:<$2>\n" : "No integer\n"; print defined $4 ? "Fraction:<$4>\n" : "No fraction\n"; } else { print "NO MATCH\n"; }'
NO MATCH
Please let me know if there's something that I have overlooked.
Addition
Enforcing braces is trivial, just wrap the whole thing in ESCAPED parentheses:
^\((?=.)([+-]?([0-9]*)(\.([0-9]+))?)\)$
