Pattern matching with the single character () wildcard in Java

Drawer QR Code ISO/IEC18004 in Java Pattern matching with the single character () wildcard

The simplest way to search for a pattern is to look for a single character For example, you might want to know if a certain string begins with the letter b, ends with the letter t, and contains exactly one character between Although you could repetitively check every three-character string that begins with b and ends with t, like bat or but, it s much easier to use a single-character wildcard instead, which is a dot or period character () So if you want to find every three-letter string that begins with a b and ends with a t, you d use this regular expression:

To search for multiple characters, use the () wildcard multiple times to match multiple characters So the pattern bt matches the strings boot and boat with the two () wildcards representing the two characters between the b and the t Of course, the bt pattern doesn t match bat because bat has only one character between the b and the t Nor does it match boost because boost has more than two characters between the b and the t When using the () wildcard, you must know the exact number of characters to match

The () wildcard can find any character whether it s a letter, number, or symbol Rather than search for any character, you can also search for a list of specific characters by using the square bracket [ ] symbols Enclose the characters you want to find inside the square brackets So if you want to find all strings that begin with b, end with t, and have an a, o, or u between, you could use this regular expression:

The preceding example finds words, like bat or bot, but doesn t find boat or boot because the regular expression looks only for a single character sandwiched between the b and the t characters As an alternative to listing the specific characters you want to find, you can also use the not (^) character to tell the computer which characters you don t want to find, such as

This tells the computer to find any string that doesn t have an a or an o between the b and the t, such as but If you have the string bat, the b[^ao]t regular expression ignores it

Sometimes you may want to find a string that has a specific character, but you don t care how many copies of that character you may find That s when you can use the (*) wildcard to search for zero or more specific characters in a string So if you want to find a string that begins with bu and contains zero or more z characters at the end, you could use this regular expression:

buz*

This finds strings like bu, buz, buzz, and buzzzzzz Because you want to find zero or more copies of the z character, you place the (*) wildcard after the z character The (*) finds zero or more characters, but what if you want to find at least one character That s when you use the (+) wildcard instead To search for a character, you place the (+) wildcard after that character, such as

Wildcards can match zero or more characters, but sometimes you may want to know whether a particular character falls within a range or characters To do this, you can use ranges For example, if you want to know whether a character is any letter, you could use the pattern [a-z] as follows:

bu[a-z]

This finds strings, such as but, bug, or bus, but not bu (not a three-character string) Of course, you don t need to search for letters from a to z You can just as well search for the following:

bu[d-s]

This regular expression finds bud and bus but not but (because the t lies outside the range of letters from d to s) You can also use ranges to check whether a character falls within a numeric range, such as

21[0-9]

This finds the strings 212 and 210 If you only wanted to find strings with numbers between 4 and 7, you d use this regular expression:

21[4-7]

This finds the strings 215 but not the strings 210 or 218 because both 0 and 8 lie outside the defined range of 4 7 Table 3-6 shows examples of different regular expressions and the strings that they find This section shows a handful of regular expression symbols you can use to search for string patterns A lot more regular expressions can perform all sorts of weird and wonderful pattern searching, so you can always find out more about these other options by browsing wwwregular-expressions info By stringing multiple regular expression wildcards together, you can search for a variety of different string patterns, as shown in Table 3-6