$re = '/(\d*\.\d*)( lb \(\d*\.*\d* kg\))/m';
$str = 'Age Weight Length
0 7.4 lb (3.3 kg) 19.6" (49.8 cm)
1 21.3 lb (9.6 kg) 29.8" (75.7 cm)
2 27.5 lb (12.5 kg) 34.2" (86.8 cm)
3 31.0 lb (14.0 kg) 37.5" (95.2 cm)
4 36.0 lb (16.3 kg) 40.3" (102.3 cm)
5 40.5 lb (18.4 kg) 43.0" (109.2 cm)
6 45.5 lb (20.6 kg) 45.5" (115.5 cm)
7 50.5 lb (22.9 kg) 48.0" (121.9 cm)
8 56.5 lb (25.6 kg) 50.4" (128 cm)
9 63.0 lb (28.6 kg) 52.5" (133.3 cm)
10 70.5 lb (32 kg) 54.5" (138.4 cm)
11 78.5 lb (35.6 kg) 56.5" (143.5 cm)
12 88.0 lb (39.9 kg) 58.7" (149.1 cm)
13 100.0 lb (45.3 kg) 61.5" (156.2 cm)
14 112.0 lb (50.8 kg) 64.5" (163.8 cm)
15 123.5 lb (56.0 kg) 67.0" (170.1 cm)
16 134.0 lb (60.8 kg) 68.3" (173.4 cm)
17 142.0 lb (64.4 kg) 69.0" (175.2 cm)
18 147.5 lb (66.9 kg) 69.2" (175.7 cm)
19 152.0 lb (68.9 kg) 69.5" (176.5 cm)
20 155.0 lb (70.3 kg) 69.7" (177 cm)
';
$subst = "\1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php