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GNU grep, which supports a wide variety of POSIX syntaxes and extensions, uses BM for a first-pass prefiltering, and then uses an implicit DFA. Wu agrep, which implements approximate matching, combines the prefiltering into the DFA in BDM (backward DAWG matching). NR-grep's BNDM extends the BDM technique with Shift-Or bit-level parallelism. [54]
grep is a command-line utility for searching plaintext datasets for lines that match a regular expression.Its name comes from the ed command g/re/p (global regular expression search and print), which has the same effect.
/S Searches for matching files in the current directory and all subdirectories. /I Specifies that the search is not to be case-sensitive. /X Prints lines that match exactly. /V Prints only lines that do not contain a match. /N Prints the line number before each line that matches. /M Prints only the filename if a file contains a match.
Conversely, for simpler operations, specialized Unix utilities such as grep (print lines matching a pattern), head (print the first part of a file), tail (print the last part of a file), and tr (translate or delete characters) are often preferable. For the specific tasks they are designed to carry out, such specialized utilities are usually ...
The command has a number of switches: a - number all lines; t - number lines with printable text only; n - no line numbering; string - number only those lines containing the regular expression defined in the string supplied. The default applied switch is t. nl also supports some command line options.
grep is a command-line utility for searching plain-text data sets for lines matching a regular expression and by default reporting matching lines on standard output. tree is a command-line utility that recursively lists files found in a directory tree, indenting the filenames according to their position in the file hierarchy.
A ? may be placed after any repetition quantifier to indicate that the shortest match should be used. The default is to attempt the longest match first and backtrack through shorter matches: e.g. a.*?b would match first "ab" in "ababab", where a.*b would match the entire string.
Commentz-Walter has two phases it must go through, these being a pre-computing phase and a matching phase. For the first phase, the Commentz-Walter algorithm uses a reversed pattern to build a pattern tree, this is considered the pre-computing phase. The second phase, known as the matching phase, takes into account the other two algorithms.