![]() Tcl has two more word boundary tokens that do discriminate between the start and end of a word. They don’t discriminate between the start and the end of a word. These Tcl regex tokens match exactly the same as \b and \B in Perl-style regex flavors. \y matches at any word boundary position, while \Y matches at any position that is not a word boundary. Tcl uses the letter “y” instead of the letter “b” to match word boundaries. \B matches a single backslash character in Tcl, just like \\ in all other regex flavors (and Tcl too). In Tcl, \b matches a backspace character, just like \x08 in most regex flavors (including Tcl’s). Notable exceptions are the POSIX and XML Schema flavors, which don’t support word boundaries at all. Word boundaries, as described above, are supported by most regular expression flavors. If we had used the regular expression is, it would have matched the is in This. The engine has successfully matched the word is in our string, skipping the two earlier occurrences of the characters i and s. The last token in the regex, \b, also matches at the position before the third space in the string because the space is not a word character, and the character before it is. The engine continues, and finds that i matches i and s matches s. It matches there, but matching the i fails.īut \b matches at the position before the third i in the string. Again, the \b fails to match and continues to do so until the second space is reached. The engine reverts to the start of the regex and advances one character to the s in island. This fails because this position is between two word characters. Now, the engine tries to match the second \b at the position before the l. Continuing, the regex engine finds that i matches i and s matches s. Again, the engine continues with the i which does not match with the space.Īdvancing a character and restarting with the first regex token, \b matches between the space and the second i in the string. ![]() \b matches here because the space is not a word character, and the preceding character is. The next character in the string is a space. It cannot match between the h and the i either, and neither between the i and the s. \b cannot match at the position between the T and the h. i does not match T, so the engine retries the first token at the next character position. The engine does not advance to the next character in the string, because the previous regex token was zero-length. The engine continues with the next token: the literal i. \b matches here, because the T is a word character and the character before it is the void before the start of the string. Since this token is zero-length, the position before the character is inspected. The engine starts with the first token \b at the first character T. Let’s see what happens when we apply the regex \b is \b to the string This island is beautiful. Effectively, \B matches at any position between two word characters as well as at any position between two non-word characters. \B matches at every position where \b does not. So saying “ \b matches before and after an alphanumeric sequence” is more exact than saying “before and after a word”. This regex does not match 44 sheets of a4. Since digits are considered to be word characters, \b 4 \b can be used to match a 4 that is not part of a larger number. Using only one operator makes things easier for you. This is because any position between characters can never be both at the start and at the end of a word. Most flavors, except the ones discussed below, have only one metacharacter that matches both before a word and after a word. ![]() Java supports Unicode for \b but not for \w. In most flavors, characters that are matched by the short-hand character class \w are the characters that are treated as word characters by word boundaries. All characters that are not “word characters” are “non-word characters”.Įxactly which characters are word characters depends on the regex flavor you’re working with. A “word character” is a character that can be used to form words. ![]() Simply put: \b allows you to perform a “whole words only” search using a regular expression in the form of \b word \b. Between two characters in the string, where one is a word character and the other is not a word character.After the last character in the string, if the last character is a word character.Before the first character in the string, if the first character is a word character.There are three different positions that qualify as word boundaries: It matches at a position that is called a “word boundary”. The metacharacter \b is an anchor like the caret and the dollar sign.
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