Javascript正则表达式-仅匹配一组4个数字

Question

I'm new to regex and and having trouble coming up an expression (javascript) to match what I need.

For this example:

21 Apr 1789 -  4 Mar 1797  John Adams                        (b. 1735 - d. 1826)  Fed 
 4 Mar 1797 -  4 Mar 1801  Thomas Jefferson                  (b. 1743 - d. 1826)  D-R 
 4 Mar 1801 -  4 Mar 1805  Aaron Burr                        (b. 1756 - d. 1836)  D-R 
 4 Mar 1805 - 20 Apr 1812  George Clinton                    (b. 1739 - d. 1812)  D-R 
 4 Mar 1813 - 23 Nov 1814  Elbridge Gerry                    (b. 1744 - d. 1814)  D-R 
 4 Mar 1817 -  4 Mar 1825  Daniel D. Tompkins                (b. 1744 - d. 1825)  D-R 
 4 Mar 1825 - 28 Dec 1832  John Caldwell Calhoun             (b. 1782 - d. 1850)  Dem 
 4 Mar 1833 -  4 Mar 1837  Martin van Buren                  (b. 1782 - d. 1862)  Dem 
 4 Mar 1837 -  4 Mar 1841  Richard Mentor Johnson            (b. 1780 - d. 1850)  Dem 
 4 Mar 1841 -  4 Apr 1841  John Tyler                        (b. 1790 - d. 1862)  Whg  
 4 Mar 1845 -  4 Mar 1849  George Mifflin Dallas             (b. 1792 - d. 1864)  Dem 

What I need from each row is only the 4 digit year that comes after the "b. ". So if I returned only my matches, it would be:

1735
1743
1756
1739
1744
1744
1782
1782
1780
1790
1792

No matter how I try to build the expression, whether on my own or using any of the online web apps, it always wants to match on all the 4-digit years, or, all 4-digit years up to and including the one I need. This is certainly due in part to lack of experience on my side, so any help would be greatly appreciated.

Answer 1

Using the regular expression /b\\. (\\d{4})/ /b\\. (\\d{4})/ should match all of the birth dates in the list.

Answer 2

You can use this regex: b\\.\\s(\\d{4}) and access capture group 1 to get the values you want.

Try it here: https://regex101.com/r/zxbrs0/1

Answer 3

The easiest way to do this (in Javascript, since you tagged Javascript on this question) is simply to do two regular expressions. For example, you could do this.

var str = '4 Mar 1797  John Adams                        (b. 1735 - d. 1826)';
var regex = /\(b. [0-9]+/;
var regex2 = /[0-9]+/;
var birthDate = regex2.exec(regex.exec(str));
return birthDate;

If you get the general idea it's not hard to adapt it to your circumstances. Just use a regex to find (b. 1735 (in this case) and then extract 1735. This is only necessary because, unfortunately, regex in Javascript doesn't allow for the same look behind feature that other regex engines do. In another platform you could use (?<=\\(b. ) to just look behind the b.

I've tested this script in Javascript, so it should work.

Alternatively you could also use capturing parentheses. Try this in place of the regular expressions and var birthDate above:

var regex = /\(b. ([0-9]+)/;
var birthDate = regex.exec(str)[1];

Just remember that exec() should return an array so you'll need to specify that you want the second element ([1]).

This is also tested.