Post 6: Using Regular Expressions (Regex) in Vi Editor

1. What Are Regular Expressions (Regex)?

Regular Expressions (Regex) are text patterns used to search, match, and manipulate strings in a file.

They allow Vi to:

Find specific signal names or patterns
Filter simulation errors or warnings
Modify repetitive code or log lines automatically

Regex is the secret behind Vi’s smart searching — it transforms a plain editor into a mini automation tool.

2. Basic Regex Symbols in Vi

Symbol	Meaning	Example
^	Beginning of line	/^module → Matches lines starting with “module”
$	End of line	/endmodule$ → Matches lines ending with “endmodule”
.	Any single character	/b.t → Matches bat, bit, bot
*	Zero or more of previous character	/clk* → Matches cl, clk, clkkk
[abc]	Match any one of a, b, or c	/[ABC]_done
[^abc]	Match any character except a, b, or c	/[^abc]data
[0-9]	Match any digit 0–9	/data[0-9]
[a-z]	Match lowercase letters	/sig[a-z]
[A-Z]	Match uppercase letters	/CLK[A-Z]
\|	Logical OR (alternation)	/fail\|error
\.	Literal dot (.)	/version\.1
\*	Literal asterisk (*)	/clk\*

3. Examples of Regex-Based Search

Pattern	Matches	Use Case
/^set	Lines starting with “set”	Searching constraint commands in .sdc
/;$	Lines ending with “;”	HDL syntax validation
/clk[0-9]	clk0, clk1, clk2…	Checking multiple clock nets
/reset_n\|rst_n	reset_n or rst_n	Finding both reset signals
/^//	Commented lines	Filtering config comments
/error.*timeout	Lines with both “error” and “timeout”	Debugging simulation failures

4. Regex in Substitution Commands

Combine regex with substitution for bulk text automation:

Syntax:

:[range]s/pattern/replacement/[options]

Example	Description
:%s/^#.*//	Remove all commented lines starting with #
:%s/\s\+$//	Remove trailing spaces (cleanup)
:%s/clk[0-9]/clk_main/g	Rename all clock signals clk0, clk1, etc. to clk_main
:%s/FAIL\|ERROR/ALERT/g	Replace FAIL or ERROR with ALERT

💡 Options:

g – Apply globally in line
c – Ask for confirmation before replacing
i – Ignore case

5. Line Anchors and Word Boundaries

Symbol	Meaning	Example
\<	Start of a word	/\<data\> → matches only full word “data”
\>	End of a word	/clk\> → matches clk, not clk_main
\s	Space	/\s\+ → matches one or more spaces
\S	Non-space	/\S\+ → matches non-space text

Example:

Replace only full-word rst_n with reset_n:

:%s/\<rst_n\>/reset_n/g

6. Real-World Examples for VLSI Engineers

1. Clean Up Log Files

Remove all lines starting with # or empty lines:

:%s/^#.*//g

:%s/^\s*$//g

2. Extract Simulation Failures

Search for any line with “fail”, “error”, or “timeout”:

/fail\|error\|timeout

3. Rename HDL Signals

:%s/\<data_in\>/data_bus/g

:%s/\<rst_n\>/reset_n/g

4. Align Module Names

Find module declarations only:

/^module

5. Cleanup SDC or Tcl Scripts

Remove trailing semicolons and fix spacing:

:%s/;$//g

:%s/\s\+/ /g

7. Combining Regex with Ranges

Apply substitution to specific line ranges:

:100,200s/^set_input_delay/set_delay/g

Changes text only from line 100 to 200 — ideal for editing large timing files.

8. Multi-Pattern Search Tricks

Task	Command
Search for error or warning	/error\|warning
Search for “fail” followed by “warn”	/fail.*warn
Search for line starting with set and ending with clk	/^set.*clk$/
Search for 3-character signals starting with a	/^a..$/

9. Debugging Regex in Vi

You can preview what matches before replacing:

:g/pattern/p