One of the most common ways of doing this is by using groups in the replacement expression. For more info take a look at the grep manual page by typing "man grep" in your shell.Ī lot of the time you'll want to change a line subtly, rather than replace static text with completely different text. Grep is an extremely handy utility for searching in text files using regular expressions, but be careful, the syntax for grep is not 100 percent identical to what perl uses. In fact, the previous command could easily be replaced by: When I run this as soĬat /usr/share/dict/words | perl "h l"Ĭat /usr/share/dict/words | perl "ab lu y"Ĭommand line aficionados may notice that we've just implemented a very stripped down version of the common utility "grep".
Running quickly through this example: first we take the first command line argument, then replaces all gaps with periods, then uses this as the pattern in a regular expression match, filtering standard input for lines that match the pattern. Virtually all UNIX based systems (eg Linux and Mac) come with a reasonable word list, usually found at /usr/share/dict/words, but Windows users can pick one up here.Ī perl program to solve this task could be written like this: We want a program which takes in incomplete information about a word and then searches a word list for possible solutions.
We'll now look at a simple command line utility to help you cheat at crossword puzzles. Now let's look at how you can use these regular expression tools in a real program. Translation is a simple operation, there's no way to handle repetition or grouping, so it's suitable only for basic replacements, for anything more substantial you're better off with a series of substitutions. Handily, the second list wraps around, allowing us to write an expression like: Translation works on a per character basis, replacing each item in the first list with the character at the same position in the second list. To properly replace all lowercase vowels with their uppercase equivalent, we can use another method: the translation tool:
Instead this will replace every vowel with the string "".
Square bracket notation does not work in the replacement side of the substitution, since in general there would be no way of knowing which character should be inserted. We can't do something like the following however, if we were looking to make all vowels uppercase. This replaces anything matched by the first expression, ie: anything except a digit, with what's in the second expression, which is empty.
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